Where do you intend to put the blockers in the chain?
Between the patchbay and the source would protect the source, but I would think the P48 could still back-feed to all the decks plugged into the patchbay. You might need blocker for each of the balanced outputs on the patchbay.
FWIW, I have the same patchbox you have, although a generation or two older. When I bought it used off of ebay, two of the XLRs outputs were dead. Upon further investigation, I found that there were some resistors burned open on the circuit board, an easy fix. I always suspected that someone plugged into it with 48P on and toasted those outputs....
http://www.henryeng.com/pdf_MANUALS/PIIM.pdfYou have me thinking about modding my patchbay to choke any accidental P48 on all the balanced outputs. There is plenty of room to add some circuitry inside those Henry Engineering boxes...
Fortunately, 99% of my patchers plug in with RCA's so phantom isn't an issue.