I suspect an answer sandbox is a little too far off the beaten path to get attention when it's needed. If you're drafting an answer, it's probably because people's attention is on the question (and its other answers). What prompts somebody to then go look at the sandbox for answers in progress?
On the other hand, somebody coming to the answer sandbox needs to do some work before he can really look at a post there -- you've got to at least go read the question and maybe the other answers, and then come back and read the proposed answer, and then provide feedback.
It seems like people are comfortable just posting their answers -- which is great; that's what we want people to do! If somebody wants help on an answer before posting, then I'd suggest coming to chat, where there tend to be people around (or will be within hours) and where you can have more active engagement. That seems better than posting a meta answer that might go unnoticed for much longer and then have slower conversation in comments.