I dont think powering the device with android tablets is a real issue. An adapter with a 5v supply connector is easily designed. There are hdmi interfaces that use this concept. I too would buy a picoscope if I could use it with my android tablet and an additional power adaper would not break my budget. How about you do the SW porting and I design the adapter?
Yes, using a power adapter is fine. However one of the expectations of using a tablet with the PicoScope would be about achieving true portability, so a solution that does not require mains power would be better, and that is where the difficulty comes in.