FAA

Yes, the 91.205 required equipment is specific
for "powered aircraft"

But the 61.57 IFR recency requirements are pretty
demanding. 3 hours of glider instrument flight experience
in the past 6 months? Better buy your safety pilot
a nice dinner...

There's not much uncontrolled airspace suitable
for cloud flying, though. There are pilot certification and recency
of experience requirements in Part 61.

the word "much" is relative. G airspace goes quite high
near Reno in some areas. Anywhere ATC cannot get a reliable
altimeter reading, the G airspace may go into the low
teens. These "fences" of airspace are likely candidates
for this kind of flying. I'm guessing this is where
Carl Herold did his IFR flying...

This has been a VERY enlightening discussion...