As for being able to mine at all beneath the Dust threshold, possibly pool-side monitor what current payout threshold based on pool THs vs any single users percentage of that. If a user goes below threshold at least auto-send them a canned email about it?
Assuming a 0-transaction fee block, after 0.9% fee: 12.5 x 0.991 = 12.3875btc
0.0001 / 12.3875 = 0.0008073% of pool rate.
At today's 87PH pool rate, this threshold is about 0.7TH/s, discounting the ramping period.
I see that the complexity centered around the indeterministic timing of block found. In practice, it doesn't matter how much hash rate one has, there still exists the possibility of mining dusts during ramping period.