So I was talking to a friend of mine who is an EE (who does some audio design) about this conversation, and he indicated that if JRiver's filters are all minimum phase (which I believe they are), a given frequency response curve completely dictates the phase. So to bring it back to the previous point, if you use a series of minimum phase filters to approximate the slope of a Bessel filter, it will have the same phase characteristics as a Bessel filter, and so will be, for all meaningful purposes, a Bessel filter. Of course, all that goes out the window when you're discussing non-minimum phase filters (i.e. linear phase filters)
So if my friend is correct, JRiver (in the active crossover context) is even more flexible than I was giving it credit for. You could (with adequate math background, visualization skills, or trial and error) use the tools in PEQ to make any conventional crossover topography (or even unusual ones like Horbach-Keele).
That linear phase software you dropped in above is incredibly easy to use, and provides the ability to effectively "linearize" your existing crossovers (and box-based phase ripples too!). I've spent the whole afternoon A/Bing the same set of EQ/crossovers, switching back and forth from linear to minimum phase. I was surprised at how immediately audible the difference is (at least one of my guests could immediately identify which was which with her eyes closed).
I can't decide which one I prefer, but I really need to do some additional measurements before I get too far down that road.