In order for Trisquel to run on a Raspberry Pi it would have to include
non-free firmware, infringing on user freedom and becoming ineligible for FSF
endorsement. https://www.fsf.org/resources/hw/single-board-computers
The only libre replacement for the Raspberry Pi that I'm aware of is the
Libre Tea Computer Card, which runs Parabola. Trisquel could run on an EOMA68
computer card as well if it had support for ARM.
I don't know what all is required for Trisquel to support ARM. I assume it
would be easier if the upstream Ubuntu version did, but as far as I know only
Ubuntu Server does currently. The last Ubuntu Desktop release with ARM
support appears to be 12.04
