Command modifiers can be used to change your command before running:

let g:asyncrun_program = get(g:, 'asyncrun_program', {})
let g:asyncrun_program.nice = { opts -> 'nice -5' . opts.cmd }

When you are using:

:AsyncRun -program=nice ls -la

The command ls -la will be changed into nice -5 ls -la.

The -program=msys, -program=wsl are both implemented as a new command modifier it changes command ls into:

c:\windows\sysnative\wsl.exe ls

And replace any thing like $(WSL_FILENAME) and $(WSL_FILEPATH) in your command.