If both entry and exit are at ungated stations then the risks with paper
tickets and Oyster seem pretty much equal. In theory one could touch in
on exit if ticket checks are seen and take the 'penalty' hit rather than
get caught travelling fraudulently, but in practice RPIs can watch the
validators and see this happening.

Isn't the amount charged on entry at NR stations such as Marylebone, where
Oyster is in use, already more
than the 'tube' version, and broadly the same as a single NR journey to
zone 6?

I seem to remember it is £5 but what that equates to I don't know. However
it does seem to suggest that the system is flexible enough to charge
different amounts on entry at different stations and this amount might need
increasing a bit to allow for travel to ungated stations way outside the
zonal area.