We are indeed going to disallow it, for the second reason that you mention, TEIN. It could influence the voting. People might honestly and earnestly try to put the longer version out of their minds as they decide who to vote for in the 75 Word Challenge, and still not succeed in eliminating a subconscious influence.
However, we're still thinking about whether it would be all right for someone to post a 300 word story based on one of the 75 word stories from an earlier month. At that point, there is no influence on judging the 75 word story (because it's already had its day), and it's not a bad idea to encourage people to take their unfinished ideas and expand on them.
*****
I have given up my ambition to write my story in such a way that it would make strong men weep, and just gone ahead and posted it as it is.