MXUnit has a method addAssertDecorator() in Assert.cfc. This is for adding-in custom assertions at runtime.
I cannot use TestBox without this functionality (I also need to be using MXUnit compat at the moment... am not in a position to move off it yet).
NB: I've raised this as a blocker, but only in the context of MXUnit Compatibility, not "in general". I dunno how you like your priorities set for these things. Feel free to change it