I've read that any keyboard key can be recognized by the simulator, but what about hardware? Can a usb keyboard be plugged directly with a usb-c adapter? Can Mirror send keypresses to the hardware when Mirror is released?
I believe they talked in this video about how Mirror could be used to use your keyboard (or other input devices) with your Playdate.
That doesn't answer your question about peripherals connecting directly to the Playdate, but I hope that helps! 
Thanks; more specifically, it is discussed here: https://youtu.be/papAhTwneh0?t=76
I am wondering whether I can detect every letter of the alphabet being pressed tho, not just whatever is mapped to the playdate's buttons. So: can I do full text entry with a keyboard?
And then there is the question of connecting a keyboard directly to the device
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Keyboard cannot be plugged into the hardware, but a keyboard connected to the same PC as the Playdate can type most keys (any key that can be expressed as a string) using playdate.keyPressed(key), which is meant for debug purposes but can be used otherwise.
A good feature request for the Playdate Simulator would be to allow control remapping?
Playdate Mirror app has that feature already.