Originally Posted by NocoRider

Not entirely clear on your intent but some things to consider... (food for thought)

The 1x12 derailleur will work as a friction derailleur BUT shifting between more than 7 or 8 cogs requires a lot of precision. So a 1x12 friction drivetrain would require accurate shifting and trimming.

The freehub body for a 12 speed cassette is much wider than 8 speed which is wider than 7 speed. SRAM 12 speed cassettes require the XR freehub body which would not fit on any of the vintage hubs. Shimano 11 speed cassettes have the same splines as 7 speed so maybe one could fit 9 or 10 of the 11 speed cogs onto a 7 speed freehub???

12 speed chainrings & cogs are MUCH narrower than the 7 speed chainrings & cogs so the 12spd chains are much narrower. The derailleur's jockey wheels are designed to match the chain width. So putting a 12

The pull ration of 12 speed derailleurs is incompatible with 7 speed shifters - indexing would not work.