Wow, Richard, didn't mean to step on your toes. Of course, any framebuilder can become a constructeur, and many of the parts were for "coolness" reasons. But then, lugs aren't necessary eiter. I am sure there was a perceived "reason" a Singer fillet-brazed stem was better than an aluminum Pivo or Philippe... But mostly, it just was part of the bike. Singer is a good example (stem excepted), because they pretty much used what was available. They stopped using their own brakes once the superior Mafac Racers came along. Same about front derailleur - starting more or less in the late 1950s, they used Huret (so did Herse).
That said, a frame/fork kit won't make a good randonneur bike.
Fender attachment needs to be carefully considered, otherwise they won't follow the curve of the wheel gracefully. So a randonneur bike should come as frame, fork and fenders. Racks, too, because elegant racks are custom-made, rather than adjustable. Now brakes - they need to be chosen carefully so they don't interfere with the racks and fenders. You see, we are getting to a complete bike rather quickly.
Nowadays, Singer, Berthoud and a few others still work as "constructeurs." They use mostly existing components - Singer still make their stems. So no, you don't need to make your own parts, although the golden era for ready-made parts were the 1970s, and a lot of stuff is posing problems these days, most notably hubs (no more Maxicar that allow spoke replacement on the road), lights (none for brazed-on fittings) and brakes (most of them don't have clearance, and a fender that moves when the brake is applied won't do, sorry.)
In their time, there were at least 2 dozen constructeurs all over France. Most made a few parts, nobody as many as Herse. I'd say the few years after WW II until 1955 were the best time, because bikes were a status symbol, so there was a large market. With the advent of cheap cars, bikes became the opposite, a symbol of poverty and backwardsness. Only a few diehards continued to ride and buy expensive bikes.
Makers I can think of right now, besides Herse and Singer: Jo Routens (stem, front derailleur, racks), Goëland (racks, pedals?), Narcisse, Faure, Maury, Daudon, Baras (aluminum frames, aluminum racks). And there were many unknowns, like the maker of my Ondet/Lyon.
In other countries, I can think of Jack Taylor/Britain (who used French parts for his touring bikes), Mariposa/Canada. Italy seems to have been focused on racing, although I have heard of a few early touring bikes. Germany has never produced much of note. Any other bikemaking country? I know there were a few people making French-style bikes in the U.S. in the 1970s. Anybody got more info?
Look at a Rivendell, and you realize why it is hard to make a "randonneur" bike with ready-made parts. The bike may work great, but it just doesn't look elegant. Can't look elegant, with clamps, zip-ties and other accroutements. So most people ride them without fenders and racks, you are back to a racing bike - sort of like a Cinelli Supercorsa.
A randonneur truly is a unit - you don't just take a part off and replace it with another. That can be a drag (say the front derailleur breaks and no replacements are available that fit the special braze-on), but the benefit is a bike that works, because each part is designed to work together. Also, it was common to have a bike updated once in a while, so you'd take it back to Herse, who would put the newest derailleurs on and a new paintjob, too.
The frames of Herse, etc., were nicely made, but ornate lugwork was not their thing. The elegance came from being extremely functional, not from a swirl and a curl here and there. Maybe the cheap Nervex lugs spoiled that - cheap bikes could look ornate, so the "constructeurs" had to come up with something else to distinguish their wares. Instead, the tubing was carefully selected to match the rider and purpose. Herse went further than most and used a lot of oversize tubing. How do you like a 1950 tandem (open frame) with a 32 mm downtube, 30 mm top tubes, and I forget what diameter seat tubes. No wonder the thing rides wonderfully despite the lack of bracing, unlike other tandems from the era.
Just as an example, on my beloved Rivendell, today I filed off part of the Lumotec front light, because it was very close to the spokes. The light is mounted on a home-made stainless bracket on the front dropout. It was never intended to be mounted there, but the standard location (in front of the brake) is taken up by the handlebar bag. Plus, the standard bracket (cheap pressed steel) broke during the last PBP (when I didn't use a handlebar bag yet), so a better solution had to be found. (I should have asked the maker of the rack to include a mount for the light, but I prefer a lower beam that is horizontal and illuminates distant road signs, for navigation.) So I basically became a "constructeur" when I modified the light to fit on a home-made bracket. Same with the rear lights - I don't want something that is clamped on and starts to slip in the middle of an event. (Last year, I rode S2S, our cross-state race, with two guys, both of whose rear lights had moved and were shining at the pavement.) So I had to make my own brackets, put on braze-ons that I specified to the builder. You want to hear more...? You see, I'd rather not bother and get a bike that is ready to ride.
This is not to devalue racing bikes, which are light, beautiful and great fun to ride. I have several... They just are not my choice for 400 km or more in the rain without stopping. Nor with 50 lbs of luggage around the Andes. For the latter two purposes, there are specially conceived bikes, and they do the job well.
Jan Heine, Seattle