I’ve been looking for a good, small monitor for my Raspberry Pi 3B, something of the size and resolution of a Sumsung Galaxy so I can have the option to attach it to a VR headset (for privacy and viewing size). I’ve read ways you use apps to stream the Pi’s video to an Android phone, and I hope those will work with the Librem 5 and Purism OS, but I’d like to suggest that adding a Type C HDMI input to the Librem 5 (+?) would open an unexplored and potentially very large market to the phone.
while this is a pretty neat idea I have to disappoint you, no, this is not going to happen and even AFAIK impossible to implement. ATM I am only aware of a specification of HDMI or eDP output via the alt mode configuration of the USB-C interface, input is AFAIK not (yet) specified. And even if you would do that this would require a huge amount of extra parts. The internal display of the L5 is directly connected to the MIPI-DSI port of the CPU, i.e. the i.MX8M. To input a HDMI signal it would require some chips to decode the HDMI data stream into picture frames again, push these into the CPU so that the CPU can push these our to the MIPI-DSI again. I do not want to suggest that this is impossible but it would require a lot of extra chips, power consumption, PCB space etc. which is very unlikely to be available - also not on a V2, V3, …
I would suggest to look more closely into the software driver approaches.