It was not the intention to power SEQ v4+ from MatriX, but if you really wanted to try it, it could work. If you try it is at your own risk!
On SEQ v4+ MIDI8: +5V is connected to pin 8 of the DIN8 plug (this is the single pin on its own row in the middle). This +5V would only be connected into the SEQ v4+ +5V domain if the wire jumper is bridged, then the power would flow back through the IDC J1. Instead, it would make more sense to populate the Molex 3-pin header and use thicker gauge wire. Pin 1 (closer to the 220R resistor) carries +5V and the middle pin is 0V. This could then be connected to other Molex headers, e.g. on the USB board or even directly to e.g. lemec_R J7. Note that lemec J1 also uses the same footprint (with smaller solder pads) and rather is for connecting a pair of LEDs.
On MatriX: the DIN pin 8 is not connected, but it could be bodged e.g. from the J5 header or the switching regulator itself. Please make sure not to connect +12V here!
The older BLM 16×16+X didn’t work that well when powered over the DIN header. There was significant voltage drop/droop in the cable so much that I found it best to install a boost regulator to get proper +5V in the case. Thus although more cables are needed, I recommend to use separate power supplies for each device. Your mileage may vary!