Thanks John, someone posted that there have been some developments, like an update to the Jecklin design, now 35cm. I wonder if instead of a disc something more like a human head's dimensions might be worth a shot, made of dense open-cell foam. It's not difficult to make something like that to experiment with. Without going to the Neumann binaural head extent, just something approximating the width and vague profile of the human noggin but shaped like a wheel of cheese, if that helps picture it. A thing that looks like a dead long-haired cat wouldn't look good at the conductor's position, say for the London Symphony Orchestra at a major concert.
I bought a stereo bar with 4 mounting points so I can have a pair of omni mics at 40cm spacing and something else in X-Y or ORTF on the inner pair. The omnis will be Oktava capsules on their mini-pre's, I'll experiment for the middle pair (I quite like MK-102 or MK-103 end-address LDC's too). Each pair will be there in the hope that at least one yields a decent result, or to be a backup for the other.
After buying other equipment recently I don't have the budget for KM84, DPA, Schoeps etc so Okkies it is for now. I would like to experiment with a Jecklin or Schneider disc (or foam cheese wheel shape... call it a Jarlsberg disk

) and those mics seem like a good starting point for me, until I've got enough such recordings under my belt to know what techniques I'd use with higher-end mics.
If this isolation disc technique does turn out to be hit-and-miss then good old A-B with omni mics would be a good system to default to, at least I know it'll usually give a result I can use. There's a Jecklin and a Schneider listed at Thomann but I could make variations to try out.