even fitting the connect to 1 single standard theo mount, there was a lot of play.

you may have an issue with play on your idea, mine needed the bush at the back to ensure the set up was rock solid, even the dovetail rails have movement, that is why I never use anything other than Rapid mounts on the Rapid, once they are both tight they should (and have been) true to the bore line.

Anyway, drilling and tapping the block at the top, isn't that big a deal, it is covered by any normal scope, you could put a blanking screw in there and you also have the advantage of other mounting options, ie a reach back mount from this position, reach forward mount from this position or just a standard mount facing one of two ways.