I want to .include a file from the same directory as the .sch I am working on, but so far haven't found a way to use relative paths so this would work. Full path names work just fine, but these break if I move the files elsewhere. Relative paths are resolved relative to where xschem is installed and not where the .sch is. I imagine there is some configuration option I am overlooking?

the simulation runs in the simulation directory (default; ~/.xschem/simulations) . So the relative paths start from the simulation directory. You can change the place where xschem generates the spice netlist or you can also force xschem to put the spice file and run simulation in the same directory of the schematic. See Simulation menu entry:

Thanks! That gets the job done, though I'll look more closely at the other options to see if they might be more convenient