Thanks for posting the Sabine Gould link...not come across that book before, currently downloading!
Tolborough does have an 'entrance' on the south side, but it takes you up onto the top of the cairn. ...I think a little artistic license here.
As for Brown Willy, yes it does sound like Rilaton..the cairn on Brown Willy being just a pile of stones.
If you come across any other Bodmin Moor nuggets..or novels..please keep me informed.
all the best
Mr H