Customizing the AR Transaction form by creating a custom value set to support an open text field

Hello,

I'm trying to figure out the best way to validate the Customer Reference field on the Reference Information tab of the AR Transaction form against a custom value set.  The Reason field above it is supported by the CREDIT_MEMO_REASON lookup and I'd like to relate the fields together with a limited list of values to be made available under the Customer Reference field...which is an open text field by default.  Any suggestions from anyone who's done this before would be greatly appreciated.

Thanks,

Phil