Per Section 10.9.2 of the UEB codebook, shortforms for deceiving, said and rejoice can only be used within other words when the longer word appears on the Shortforms List given in Appendix 1. Deceivingly, unsaid and rejoiceth do NOT appear in Appendix 1.
HOWEVER, since the codebook came out, the ICEB code maintenance committee has approved the use of the shortform for said in the word unsaid (approved March 2015). Any new publications regarding UEB should reflect that change. In the meantime, I would suggest you write it into Appendix 1 of your codebook.
While making that change to Appendix 1, add the word unrepaid to the list for using the paid shortform. This change to Appendix 1 was also approved in March of 2015.