Currently Primecoin protocol allows searching for a header hash of arbitrary factorization. In fact the reference miner currently finds a hash divisible by 210, instead of taking random hash values.
If a prime chain is pre-generated with highly abundant origins (abundant meaning large number of factors), in theory you could try to search for a block header hash that divides the abundant origin. This would defeat the non-reusability property of the proof-of-work. With current limit on prime size at 2000 bit this is still computationally not feasible, but I estimate that instead of 256 bit hard of the secure hash, it might be reduced to about 128 bit hard for such brute-force attacks.
Even though this is not considered a feasible attack vector right now, there is a simple adjustment of the protocol that eliminates this doubt completely. By requiring block header hash to meet Fermat test (thus basically prime), this attack vector is eliminated.
I will be testing a modified reference miner in the next couple weeks for this change. If there is no strong objection to this protocol adjustment, it will be implemented in a v0.2 release within a couple of months time. Before the protocol is implemented, there should be plenty of time for the miners on the market to adjust to this new protocol rule.
Note: this is a hard-fork change and would be implemented as mandatory upgrade with scheduled protocol switch. There is no impact to minting/transaction/wallet related functionality.
Feel free to discuss and comment here.
Update (August 16th): Reference miner update ready for review: