Thanks, Jay.
I'm planning to just bolt it securely, in a fixed position (which I have yet to finalize - that will wait until I've got the sail in hand). And expect I'll follow Chris's advice and get or make a pole.
The wood (which will be routered and varnished) is partly for above-deck reinforcement and partly to compensate for the uneven heights of the rail fittings (the aft one is longer).
The sprit will pass directly over the location for the stemhead fitting, so I plan to make a support block like K.C.'s forward one, and have the sprit pass through that with the stemhead fitting on top of it.
I haven't ordered the sail yet. The only dimensions I was given were an I of 15 feet and a J of 6.25 feet, with 85 square feet of area. According to Paul at Doyle Ploch, this will result in a 155% overlap, with the roach making it like a 180% genoa.
I haven't been able to figure out the luff and foot dimensions, but the I dimension is the same as Chris's (and presumably, yours) and the J dimension is a bit bigger.
I measured the J dimension on my boat (after looking up sail dimensions on line to find out what a J dimension is)

. My measurement is 61 inches from the leading edge of the mast to the jib hole in the stemhead, a goodly amount shorter than 6.25 feet. I'm guessing a J dimension of 6.25 feet will let the tack be far enough forward that I can fly the sail outside the foretriangle.
The 85 square foot size is probably right for my circumstances, but I'd appreciate any comments on the dimensions that people may have. Does a J of 6.25 feet allow flying the sail outside, with enough clearance for the furler?
Other than that, the only delay is color selection. I emailed Paul at Doyle Ploch to see if I could get swatches.