I feel I ought to update my post. It turns out that my shower sump re-clogged on my trip. I tried to use my zip-tie trick with no joy. I even tried using a long stiff wire in my attempt to unclog it. Again no joy, no shower. Sadly, I had to put off fixing this until returning from my trip.
The good news is that the fix was simple. I found the filter in the system and cleaned it out. This restored the usefulness of the shower sump system on my boat. It would have been nice if the location of the shower sump pump and filter had been documented in the owner's manual or anyplace else for that matter. But alas. For the curious, it turns out that the the location of the pump and filter housing was in the v-birth, on the port side, under the mattress, under an access panel. The picture below shows what the pump and filter housing look like.