I am wondering is it possible to use alias action modifier keywords but to use the selected file's directory as an argument for the action, istead of the full filename?
Not sure, maybe there's no built in way to do that. How about this autohotkey workaround? Save as test.ahk and compile:
file = %1%
SplitPath, file,, dir
Run, cmd.exe , %dir%
Then change the alias to run test.exe with the whole filepath as alias. If you hold down shift while pressing enter on a result for that alias in FARR the script should open a command window in the files folder, if not then it will execute the file. Note: I haven't tested it, but I think it'll work.