"two padeye-type genoa cars " I recall those puppies being outright expensive. Since the mount isn't intended to be a "slider"...why not take an 18" piece of 2x4 on each side of the genoa track and simply through-bolt them together, using some heavy bolts and wing nuts? It you routed out a matching pattern so they snugged nice and tight over the track, and kept the bolts above it, you could even just lift it on/off as you loosened the bolts.
And have the ladder sit in two pockets drilled into the wood, instead of making holes in the tubing.
(A machine shop, some bar stock, whole thing could be done very nicely in all metal, too.)
I'd just be afraid that drilling holes in the ladder would lead to the holes tearing out after a while. (And did I mention, them genoa cars are expensive?)