I want to bounce this idea with you:

I think the boost feature should be nipped. What about for example introducing an optional boost tag?

That way, users won't have to manually type "$boost" anymore, but instead, configure their clients to automatically add the boost tag to their notes, which will trigger the zap-split on the note when reposted.

Additionally, the boost tag can be utilised to hold information about the split percentages.

Thoughts?