Well, I just did my test roll at 11 minutes in HC-110 dilution B, and it looks pretty good just based on eyeballing the negatives.

To zsas's question above, I didn't consider Diafine because it tends not to give much of a push/boost to slower films, and even the claim of two stops---which I think refers only to Tri-X, where the box suggests EI 1600---is a little optimistic. The MDC actually suggests a speed of 25 for Efke 25 in Diafine, which seems strange if it's really ISO 50---who ever heard of Diafine *losing* a stop?

Anyway, my plan based on this rough test is to go ahead and soup the original roll in HC-110 for 11 minutes. Thanks to everyone for the comments and suggestions.