I'm puttting my weight behind the "yes" it could be done..but tempering that with a "I doubt whether it was actually done"
There is a builder in the Midlands area of the UK who used to build seven frames a day..yes seven. OK he worked a full 12 hours, sometimes longer..and he had seven jigs....one for each size. His workshop was quite well mechanised...but he didn't have any of the Bike Machinery equipment..not until much later. I remember him telling me that once the frame parts were in the jig, it took him, on average, 13 minutes to braze all the joints...he was just accustomed to the work. The nozzle he used on his oxy-acetylene torch was about eight sizes bigger than the biggest I ever use.
His frames looked good.. but didn't have very much in the line of filing and profiling the lugs after brazing...and they rode well. He built as a sub-contractor for some of the top shops in the UK.
I have in my collection six Tour de France bikes from the 60s and 70s, two from the Sauvage - Lejeune/Lejeune team, including one of Anglade's from the 65 Tour when he finished fourth- both made by Carre, three Peugeot team bikes from the Thevenet era, and one of Jean-Luc Vandebroucke's Motobecanes. The Motobecane is by far the finest in its standard of finish and was Jean-Luc's time trial frame. As for all the others, I doubt whether a file went within three metres on any of the frames once they came out of the jig after brazing. The frames are strictly utilitarian...and were made to be ridden..to do a job..and not to be admired for the skill of the craftsman finisher.
Merckx's frames did not depart from one another in any drastic way..just a little fine tuning here and there..a nibble off the top tube or a snippet off the rear stays. If De Rosa built as many frames for Eddy as he stated, and I see no reason whatsoever to disbelieve him..there would have been as likely as not several sets of tubes on the bench pre-prepared in readiness for the final tweaking, particularly if Eddy made a habit of asking for frames at such short notice. Goodness...how long does it take to recut a mitre!
Additionally it is likely that sets of seat and chainstays would always be available..for use on other team frames as well, and there would be a supply of forks available too..as Eddy seemed to stick with a certain rake. It would not be very difficult at all to braze up a frame...nor possibly to spray it.
Where the possibilities start to break down for me is the building up and fine-tuning of the bike itself...and the logistics of delivering it, possibly to the toe of Italy..or even worse into the countryside of France..although both countries have been blessed for a long time with a multitude of small local airports.
Norris Lockley, Settle UK