Definitely an idea worth exploring !

I feel a key issue to resolve is one of immediacy; ask yourself why people would want to play this way rather than (for example) through a virtual tabletop where the action happens "in real time" so to speak. Many years ago I ran a play-by-mail game for a short while, but the reality was that technology overtook it. Why wait two or three days between turns when you can play face-to-face, by email, through Discord or videoconferencing.

The earn-as-you-play mechanism is something I find totally fascinating. To have equal starting "XP", perhaps you could create a second layer token in Hive Engine that players could delegate HP to ? Rewards could be in that token, and when a character dies, retires, or the adventure ends they could then cancel the delegation and convert the accumulated token back to HIVE.

I'm sure there are gaps in my knowledge of how all this works - the GM would probably need a decent sized pool of tokens to start with that could be allocated as XP each session, and the system would ideally be trustless in some way to minimise the risk of a DM rug-pull..