So after a bit more thought, I"m leaning towards a two-laptop solution, I should stress that this is very situation-specific...if you know Gabriel"s output, you"ll know it relies on a lot of keyboard and for some songs I run through 7 or 8 patches in mainstage, usually with a number of layers and splits within each patch. It"s much closer to musical theatre in that respect. So a laptop crash would derail the whole thing, I could cope with basic piano for a couple of songs IF the laptop could re-boot and I could get mainstage back up and running, But if not....I"d be screwed, as would the band. I"m one of those people who always expects the worst, and probably overthinks.
Adding another laptop (which I have) running mainstage with the same set of patches would probably be the only way to really be safe here. It"s closest to what I"ve seen in rundowns that are mainstage based (David rosenthals springs to mind). Carrying round an extra laptop and power supply isnt a hassle, and possibly worth the piece of mind! I mean, if I can"t get that shakuhachi sound in sledgehammer, or the bagpipes in Biko, it ain"t going to down well!
So maybe running the sl73 usb into the main MacBook, and its midi output into a second MacBook via a midi-usb interface would do the trick. Unless there"s an easier way to split the single usb out into both MacBooks?