I have contemplated trying this with a freewheel since I have done this with a cassette.

With a cassette, I just sand the plastic spacers down, or use 8 speed cassette spacers. The same can probably be done with a freewheel, but as already noted above, you have to work off of the freewheel body shoulder that separates the lowest (larger) cogs from the middle and highest (smallest) cogs. I think there are 3 cogs and I believe 3 spacers that need to go from 3.15mm to 3.00mm (the rest of the spacers also have to go to 3.00mm). Even if you use donor spacers, you will need to place a .5mm spacer behind the last cog so you can keep everything tight. Finding that spacer will be the hard part. Depending on being able to get the lock ring tight, you may also need a spacer between the lock ring and the 1st cog. It may be more work than it is worth.

Sachs Aris makes an 8 speed freewheel that can be converted to 7 speed by removing the first cog, but I don't know what cog spacing is used. I have one and haven't got around to measuring it, but I'd be surprised if it wasn't a 7 speed freewheel body with the cogs spaced at 7 speeds.

John