I just bumped into Fldigi, a digital modes client that supports the Mac (...well as a cross-platform app, not quite a full Mac app, but that's OK).
I was idly looking for a client that supports the Olivia mode, having heard a few hams waxing about this on the air, and good ol' Google surfaced it as a result. There may yet be other clients to be fished out of the great internet ocean, but I'm going to give this one a try. It seems to have a fully fledged macro system, and an auto-logger that exports adif - for import to other logging software, as well as things like eQSL. It supports a fairly wide gamut of modes; certainly most I've ever heard of, though more flora and fauna are popping up regularly. It also integrates with QRZ.com, so my subscription for web queries that I paid for in order to feed VisualQTH can keep on giving!
Anyway, I now apparently have a choice of two clients (CocoaModem and Fldigi), though I've yet to check that it will interface properly to my SignalLink and CI/V rig control. I think there's a good chance as these are both pretty common interfaces/devices. I've noticed that the IC-7800 radio profile for the rig control is labelled as "untested", but hopefully that's just the authors being conservative... we'll see.