Comment 45 for bug 287215

If your xmodmap settings are idempotent, then an awful yet effective solution to this is to use inotifywait to detect additions to /dev/input, and rerun xmodmap when this occurs.

Here's a script: https://github.com/mgsloan/mgsloan-dotfiles/blob/5efd7becaa173f2b6500aad0cc3f3724ebe22783/env/scripts/xmodmap-on-input-change.sh