After two days of searching this is turning out to be a pretty niche request, so for the benefit of future Forum searchers the options appear as follows:
File out your shifters then hope that you can get them around the bend in the bull bars
Buy some Paul’s Thumbies from SJS cycles, then convert some bar end shifters (and hope they are compatible with all the bits you’ve already purchased)
You used to be able to buy two piece drops that fitted into a big ugly stem, which would allow you to fit shifters, but these seem rarer than rocking horse…
You may also strike it lucky and find some really retro 22.2 dia. racing drops which you can convert to bull bars.
Me, I’m going with short flats and bar ends, and saving for the inevitable Thumbie’s conversion when I’m no longer bankrupt from the rest of the build…