Looks like I can now answer this, and my own questions...
Spoke to BAEC this morning, having put a flight on hold yesterday. J class availability is there on my outbound leg, but not on the return. I asked very politely if they could check with inventory management about releasing a J Class award seat on the return, and was told (very politely, but firmly) - No! Apparently inventory management will no longer release seats on request - what's available, is what's available - end of story.
I then asked whether, if I book the flight (and MFU the outbound leg for 12,500), whether if J class availability became available on the return I could upgrade that as well (for an extra 12,500). I was told that "the system can only handle one MFU per ticket, so even if availability does become available, you won't be able to use it".
So looks like I either toddle over to Virgin and use some of my miles in my Flying Club account
, or book one way NCW and hope that I'm lucky for an OpUp on the way home with BA.
If only I'd booked this two weeks ago when there were J class tixs available..................