boat came with a spinaker pole. Thinking of trying it out with some wing & wing sailing.
I have 2 halyards to choose from. One is masthead and offset to the side of the forestay. The other is for a non-existant inner forestay sail. It is about 15' from the top and centered. There is no innerforestay.
Seems like the inner forestay halyard would be a better choice for supporting the spin pole. I don't like the offset halyard next to the forestay, seems like it could get into the genoa somehow.
Any reason to not use the inner forestay halyard?
My Catalina 27 has a halyard positioned similarly to your "offset halyard"; it's the spinnaker halyard. The spinnaker pole topping lift runs through a turning block about midway up the mast and then aft to the cockpit.
The topping lift line is of a smaller diameter than the spinnaker halyard; I expect that there's typically less of a load on it. If there's enough breeze, the sail will support the weight of the pole (of course it's good to have the lift cleated in case that changes). Basically, I don't see any reason you couldn't use it; sounds just like my setup. Does your mast have a track on the leading edge whose car the pole attaches to?
As for using what I will call your spinnaker halyard to hoist the pole -- look to see where it attaches to the masthead w.r.t. the jib halyard. On mine, the spinnaker halyard is the farthest forward (which makes sense, as the spinnaker flies farthest in front of the boat. If this is the case for you, then using it to support the pole will be somewhat awkward because it will have to cross the forestay to get to the pole attachment... that's fine when the outboard end of the pole is on the same side as the halyard, put when it's on the other side, the line will chafe against the forestay. Ick.
Sounds like you want to use the lower line. I guess it's nicer to have a higher topping lift since the vertical component of its lifting force will be greater... but not significant in the face of all the hassles associated with, well, using a halyard as a topping lift.
Anyway, have fun with your winging-on-wing... let us know how it goes and what you learn (cuz we, i.e. me, are learning too).