So Smile can't help here.
It is JRiver's turn.
I said nothing about blame.
I said you'd have to ask them
how it works to be sure. MC does not use AppKit UI elements. It won't use AppKit UI elements. That's
never going to change (it means there is no MC on OSX).
So, if the way something works
requires a native AppKit UI element, then it isn't going to work. JRiver can emulate some things, but not everything the private APIs of AppKit provide, and even that would take time. And if they wanted to make this work, they'd need to know specifically from Smile exactly what they need to emulate. I don't see that technical detail here.
The same thing applies on Windows. For example, MC doesn't have native Windows 7/8 "snapping" either. They emulate it somewhat, but it doesn't work quite the same. Same stuff applies there (and has for years).
So, I don't know how TextExpander does what it does (I have and use it too, though I'd not tried to use it with MC), but if it "hooks" into the AppKit UI elements, it isn't going to find MC looking the same. If people care, the only hope would be to ask Smile for guidance, and provide it here to JRiver, if they give you good guidance. If the good folks at Smile
contact JRiver, I'm sure they'd talk to them and see if there was anything they could do to help.