Topcat,
I thought the whole point of that sort of a/c was to stay out of controlled airspace and have FUN! A single VHF comm is quite sufficient for operating into/out of the sort of airfields that you'd want to visit. GPS, as you say, for backup of your map, watch and compass, not the other way round.
We used to have fun with the Upper Heyford Mandatory Radio Area (UHMRA) with non-transponder a'c where the only significant metal was the engine
'Golf - XX you're not painting'
'No, that's 'cos we're made of wood!!'
We concluded that the Russians would invade on August Bank Holiday Monday (everyone on leave) in Jodels!
Sorry, haven't really answered your question, but I did hear the Luton controller the other day give a zone transit to a non-transponder a/c so it can happen...
Cheers,
The Odd One