Masthead sheave?
I'm curious, don't you have provision at the masthead to add an extra sheave and halyard? It would seem possible that you wouldn't need a block, just an extra shaft and sheave for the additional halyard, it might even go on the same shaft with the existing jib halyard sheave if ther is a spacer in the mastead casting. a lot of boats either already have or have at least the accomodations for a second jib halyard. What boat do you sail?