(for my use, I will probably take some wire, jump from 5v0 / 3v3 to the unused pins 7/8 so I can power from usb)
Now that I have it, a better idea (it seems to me) is just to jumper 5v0/3v3 to a 3pin header (outside ends), center pin to pin 2 of icsp connector (Vdd), then use a 2pin jumper/shunt to select 5v or 3.3.v (or no jumper for no power).
The enclosure I happened to have (Hammond 1551LTBU, $2.23ea) fits perfectly for width, is a little longer than needed but then I have room for a little perfboard for my little mod. The little rubber/silicone feet on the snap board and the case bottom pc board mounts (2) combine to keep the snap board in place (snap rests on pc board mounts, 1 pad keeps from moving as it is right up against mount).
I had to make due with what I had, so I cut a perfboard to fit the unused space, added a 8pin m-m header (bent to make right angle) so the board can be detached, added a 5pin keyed icsp connector as I will only need the 5pins and wanted a keyed connector as that makes more sense. For the 5v0 and 3v3 source points, I just put in a single pin header for each (bent at right angle) and made a couple wires with the female header connector and soldered them to the add-on board. The add-on board can be easily disconnected if I want to something different or go back to original.
I still need to make a 5pin-5pin jumper (keyed to non-keyed), and cut some openings in the case.
It does work, so will now be able to use usb power. The high side power chip they have seems to have been set to about a ~800-1000ma current limit, and I think the 3v3 regulator can handle at least that much. I don't know what the sam and the rest of the circuit consumes, but I would guess a few hundred ma for my own use should be no problem provided the usb port is willing.
One thing I have noticed- the snap likes to disconnect/reconnect from usb when power is applied to Vdd (icsp pin2), so I'm not sure what is going on. The only thing the Vdd pin does on the snap (as far as I can tell), is simply detect the Vdd voltage of the target device to adjust the other pin levels. The same thing happens to my modded snap- with nothing connected I can connect usb, then I place a jumper on my mod to get power and it will disconnect/reconnect from usb. I guess there is either something going on the Vdd detection- either causing some kind of power problem or maybe the sam is simply doing disconnect/reconnect when Vdd is detected.
edit- I'm going to also try to make a serial-telnet adapter with an esp32-wroom-32d I have so I can eliminate the usb serial-ttl I always have hooked up for high level debug/info. I ordered the wroom not realizing what I was (not) getting so have to create my own 'dev board' (from remains of my power mod board). Good thing only 6 pins needed.
I'm not sure what the best option is now for esp32, but I downloaded arduino and the arduino core for esp32 and have 'hello world' spitting out the uart.