I have added this to the todo list. Please be aware that however we're going to do this, you'll need to be able to measure both level and phase accurately to be able to configure it, because this will be a feature that's basically intended for "broken" and sub-par transmitters or sound cards. So you'll need some equipment, and for parts of the problem I don't even know how you can measure it.
For example, you would probably start with transmitting 1 kHz on one channel and listen to the other channel, and with that, adjust the phase and level of frequencies at 38 kHz.
Once that's done you can measure the tilt for lower frequencies by transmitting them on one channel as well; the difference in frequency for the L-R is very small so 99% of what you see there will be in the lows. So you can use that to configure the levels and phases of the lower frequencies.
After that it gets more difficult, for example, 10 kHz on one channel would depend on 10 kHz, 28 kHz and 48 kHz. So if there's an issue, I don't really know how to determine whether the problem is with the phase of 10 kHz, 28 kHz or 48 kHz, or all of them, and if all of them, how much and in which direction for each.
Any ideas on how to figure out when the phase is correct are welcome... Test tones, cheap measurement equipment (because if you have a bad transmitter you probably don't have very expensive measurement equipment available), etc.
What I just described is possible with the old (deprecated) correction in Stereo Tool. But it's not included in MicroMPX, and it's not trivial to put it in.