Thanks for all the replies so far. AC 43-13 is informative but doesn't give much design advice, largely falling back on 'put the fabric back as per the type design' line.
The type in question is a Jodel 117. It is covered in Ceconite but the only rib with stitching is the one at the dihedral break, probably to stop the fabric lifting off. It's such a slow speed, low wing loading aircraft that it's probably fine (been flying for >15 years since last recovered) but I just want to be sure it's OK. I have a call in with LAA engineering so hopefully they will have an answer, but if anyone knows about the matter, drop a line on this thread.
OG