I’m happy to report I was wrong… I took a chance and bought the current version of studiomux from the app store and downloaded the current (not beta) version of the studiomux Mac Server from the Zerodebug website. To my surprise and delight, everything works. I can get MIDI and audio between Rack and my iPad in every combination I’ve tried so far.
The big caveat is that I’m using MacOS Mojave (10.14) on a 2018 MacBook Pro / iOS 13.2 on an iPad Air 2. The road to here is littered with apps and devices that stopped working forever when their owners acquiesced to one of Apple’s near-obligatory one-way backwards-incompatible upgrades. There’s plenty to suggest from the reviews on the app store and the comments in the Zerodebug forum that this can happen to studiomux with some combinations of OS versions. YMMV.
As mentioned above, if you don’t need to pipe audio to & from your iPad, there may be more reliable methods. However this does work for me, and it’s something I’ve wanted to be able to do for a long time, so thanks for mentioning it, @XtremeSounds.