Geez, that's a good one. You get a star for asking a question that's probably not been asked already! ;D
There are a few policies in play here. You can only be recalled from PATA/MATA leave for an "imperative military requirement". DAOD 5001-2 lists examples of valid "imperative military requirements", such as:
- a house hunting trip under QR&O article 209.832, House Hunting Trip;
- travelling time provided under QR&O Chapter 209, Transportation and Travelling Expenses for a posting;
- Special Leave (Relocation) (CANFORGEN 054/99 ADM(HR-Mil) 042 181807Z Jun 99); and
- the period of time taken prior to a posting to complete out clearance, normally not more than one or two days.
My guess is that a CO would have a hard time justifying that an Expres test is an imperative military requirement.
As for doing the test while on LWOP - you aren't allowed to participate in the CF Sports program while on LWOP for liability reasons, therefore I would guess that the same would apply to the Expres test.
End result? I'm willing to bet you'll end up with a "not tested" dot on your PER, with the requisite explanatory statement in the narrative.
Have you checked with the PSP staff? Your chain of command?