I want to say I actually did the following but memories of it are a little fuzzy:

I put a 7 speed HG cassette, minus the smallest cog on a 6 speed UG body. I used the original threaded UG small cog to lock it all down.

I --THINK-- this is what I did. It was a long time ago and I was recovering from a badly broken leg, so painkillers were involved. If you've got the tools, this is a ten minute experiment.