In other topic people arguing now what to do if game ended incorrectly early and territory not sealed, there is a hole. I think instead of rereading old official rules and trying to find interpretation there, its better for OGS to create its own solution. We have analysis and score estimate anyway, why not create one more thing?
Currently with clicking and ctrl+clicking its possible to change status of groups, but that system has too complex mechanics and sometimes returning to game and actually closing the gap is the only way to reach score that both players wish.
Simple territory pen tool is needed. So you can click any coordinate and it becomes color that you need - black, white or neutral. And it should not matter what stones are around. So if players agree that they would place stone somewhere if they didn’t accidentally missed it, they can just draw black or white square here.