Does the law specify how long the replacement has to be a member of the party of the senator he's replacing? If not, then a Repub could simply change his party registration to D and get appointed with no legal problems.
It shows you how dumb these kinds of laws are, but my guess is that there's some elaborate rule as to how the leader of the caucus that is not of the Senator's party submits three names to the Governor, blah, blah, blah. It doesn't apply here, thankfully.