Perhaps this is already possible but I am too dense to figure it out. Is there any way to force a condition to match an empty hand (in IRuleMatchHarvesterHeldItemMainHand)?

There is now! :D

Feature available in 1.12.2-1.9.0

See: https://github.com/codetaylor/dropt/blob/master/SYNTAX.md#irulematchharvesterhelditemmainhand

Ty! That was quick :)