Will 0.5mm gap/play in a seatpost shim work without cracking the seatpost tube?
To answer the question of whether you can get away with the .5mm gap, absolutely not. It probably won't be able to clamp securely, and even trying it once could destroy the seattube opening. Bikes are essentially never tolerant of this type of mismatch.

I presume you are in this spot because you want to use a seat like a double-rail Brooks or similar that requires a pipe-style seatpost, as opposed to a more common cushy seat, most of which come with a usually-superfluous clamp that can be removed in the case of a post that has its own clamp. If that's the case, I think all you need is a 27.2-31.4 shim, which is oddbal but does exist.