according to some of the stuff i just googled, it looks like the fixing bolt for the 6600 freehub looks like any other shimano hyperglide freehub fixing bolt. if so (and i could be wrong here) you should be able to buy a 7 speed hyperglide freehub and attach it. some spacers, as you mentioned, may be necessary.
here's the pic of the fixing bolt for the 6600 that i found on several online sites (98 cents):
and as another post mentioned, a spacer before or after the 7 speed cassette will work too. i've run a 7 speed freehub with five cogs and requisite spacers without trouble.