So there will be no “legal” way to dissuade miners from an NPC corp harvesting a player-corp created asteroid field? No granting access to said field by corp/alliance membership or negotiated standings? That seems to discourage group membership and community participation, while I thought your stated aim was the opposite…
Contrary to the standard asteroid belts, these “moon fields” are explicitly owned, in the game mechanics at least – how else would you know which corp’s Mining Ledger to apply results to? So it would appear possible(*) to include similar mechanics as those for looting other people’s wrecks: giving the ninja-miner a flag and the “owning” Corporation an opportunity to respond (or not).
A reinforcement of actions having consequences, a push to community involvement, an opportunity for hi-sec PVP (including baiting by the ninja!), making ninja-mining more exciting than just sitting in another asteroid belt pressing buttons… Why wouldn’t you include such a mechanic?
_* Yeah, I know – famous last words from someone outside the development process 
_