So is there any possibility that mine pools could pick any competitive asset registration txn as…

Yes, a mine pool could preferentially select asset issuance transactions to mine. That has always been the case for blockchains, and the reason distributed mining is good. We hope that there are enough pools that no one pool could guarantee a transaction, but only increase the probability. Individual miners decide which pool to support, so they are free to switch to pools that do not accept bribes if that is in their own self-interest.