unfortunately, the internal 3V3 power supply LDO (U5, AP2210-3.3) has a maximum input voltage of 15V, so I don’t see an easy hack to make this work in the MPPT 1210 HUS.
The MPPT 2420 HC has a new circuit that selects the solar side (up to 80V for that charge controller) only if no voltage is “seen” at the battery terminal and it uses the more efficient (lower voltage) supply rail from the battery if available under normal operation conditions. I just tested this circuit before Christmas and it got a small update to make it work properly (PCB v0.2 in the develop branch). However, the feature is currently not implemented in software, so it would not yet start up if the battery voltage is below the absolute minimum. Shouldn’t be a too big change, though.