I may be unique, but I always preferred shorter movement for friction shifting. I suppose any modern (or sane) rider, or especially someone accustomed to easily and unconsciously just hurling a lever into gear (as with indexed shifting) would not be accustomed to "feeling" for a closely spaced gear, but I tend to shift slowly and attentively - certainly with no great "racing" required urgency, anyway.
As for adapting a friction shifter for increased movement, as someone mentioned already, you are likely going to also increase the full distance the lever is going to move already, simply because of the greater distance across the wider cassette (compared to a 5 or 6 speed).
~ Case in point is a 5 to 9 speed conversion I recently effected. Using existing Huret stem-shifters, I had to actually grind off a small lower stop on the lever assembly so that the movement could increase from around 110 degrees to perhaps 170 degrees. Nevertheless, this setup now works very nicely... But, now that I think of it, perhaps the ease of shifting on this particular application works better than on some because the levers are longer than typical d/t shift levers - which means your hand will be moving in a longer arc, as well.
I would be concerned about using something similar to the old Simplex "demultiplier" or a modern "Travel Agent" style cable travel increasing device precisely because it will further increase the already extended distance of lever travel - unless youre prepared to wind that old shift lever around for quite an unusual distance.
I currently use either 1970s Simplex retro-friction or mid-1980s Suntour 'Sprint' friction shifters on most of my bikes, and with anything from 5 to 9 speed hubs. In my experience, Shimano D-A 8 or 9 speed d/t shifters - even in friction mode - seem to move less distance than either of these... (maybe just my perception).
Just some considerations. Good Luck!
Bob Hanson, Albuquerque, New Mexico, USA