I think it's an interesting idea Dan. I wonder if it causes other issues. Seems like it would have been adopted by now as the J's have been doing it this way since the 90's.
I did a quick search on the rules and can't find anything specifically prohibiting it:
9. BOWSPRIT: The bowsprit and its housing shall be as supplied by the builder and may not be modified.
9.1. The bowsprit shall be retractable to a point at least equal to the stem and shall not be capable of being extended beyond 71 inches forward of the stem. When the bowsprit is fully extended, the tack line shall be on the centerline of the hull extended.
9.2. The bowsprit shall be fully retracted (so that its tip does not extend beyond the stem) when the spinnaker is not flying, except when in the process of CONTINUOUS HOIST or immediately after dousing the spinnaker.
This section talks about replacement of lines:
2. Blocks, eyes, cleats and fairleads associated with the mainsheet, jib sheets, spinnaker sheets, spinnaker halyard, spinnaker tack line, and jib furling arrangements may be replaced, but the location of the fittings shall not be changed. Additional cleats and blocks may be added. The spinnaker halyard block and the spinnaker halyard exit block may be replaced with a single halyard exit block.
It seems it would be legal to replace the current retractor line with a bungie version.