Any machinists with "spare time", take note:

What I investigated, all-too-briefly, is the possibility of using a contemporary PowerDome cassette on a freewheel body.

One would seemingly only have to enlarge the spline diameter in the flat-plate aluminum "driver disc" in order to engage the large-cogs spline on the freewheel body, then remove one or two of the smallest of the 10 or 11 cogs, then adapt a threaded cog or custom lockring to the small end of the resulting, shortened cassette.
For those unfamiliar with the PowerDome cassettes, the entire cassette is a machined, hollow dome with cogs machined into the outside. All power is transmitted through a removeable 2-d driver disc made of perhaps 5mm aluminum plate, which engages the non-driveside end of the freehub body spline.

The PowerDome cassettes are very expensive, but I've kept a few that customers were done with and which had only modest wear. They are made by SRAM, for use with their top-level gruppos (both road and mtb).

One might expect to possibly have to smooth over some of the protruding steps on a typical freewheel body in order to clear a PowerDome cassette, but the larger ones would likelyhave more clearance.

I am almost expecting to see that some entrepreneur comes up with an adapted PowerDome for use on some particular freewheel body, hopefully a Shimano 600 or lower model.
It would then be possible to fit a complete modern drivetrain along with some nice Nuovo Record or other fine freewheel hubs.