I log in to the root account in a terminal, press a deadkey combination, and nothing happens. Deadkeys work for the regular user account in the same terminal.

I'm not sure whether it matters, but: this is in lxterminal on Trisquel, an Ubuntu-derived distro (I re-use my xorg.conf and I've never had this problem on an Arch-derived distro), my input driver is lxinput, the keyboard layout is customized, a slightly modified version of Colemak, and the root account also ignores keystroks and improperly displays characters in the other languages I have set up in my xorg.conf.

I suspect this has something to do with Trisquel's obsession with using only free software. Some hardware won't work properly without a blob or two in the kernel.

If the keyboard works in a user account, it can't be a hardware problem.

What's happening is that your keyboard configuration in the user account is not being used for the root account. That's a new one on me — I have the same layout after using both "su" and "su -l". The solution is probably to copy the keyboard configuration file from ~/ to /root.

My keyboard configuration file is in /etc/X11/xorg.conf.d/

The root account is using the correct layout as far as basic keystrokes; only the deadkeys have no effect.

I am using doas instead of su...that might be relevant. If I can remember my root password, I'll see if I have the same problem with su.

You can check the components of the driver with
See if you get the same result as root and user.

In the root account I get

That reply is very strange and explanation above my pay grade. I can only suggest you join the Trisquel forum and ask there.