Is it possible to run Psychtoolbox or PsychoPy on Raspberry Pi 2 for simple perceptual experiments?
Speaking for the PsychoPy team, I would expect it to be possible but performance won't be great.

Both PsychoPy and OpenSesame use Python under the hood and this is an interpreted language. That means it isn't super-fast to execute code but we can get away with that by having fast computers (and hardware-accelerated graphics makes a huge difference).

Another option to consider might be a barebones PC like the Intel NUC computers. You can get one of those for £100 (but you have to add memory and hard disk so maybe £180 all in?). It still won't be as fast as a high-spec computer but should outperform a Raspberry Pi

Jon (for full disclosure, I'm the creator of PsychoPy)