Thanks for this walkthrough, Chris, very useful!

I guess we should document this a bit more, what do you think?

If /run/current/system/profile/lib/udev/rules.d is not used, couldn't we remove
it to avoid confusion?

Then mention the

--8<---------------cut here---------------start------------->8---
# cat /proc/251/environ | tr '\000' '\n'
...
UDEV_CONFIG_FILE=/gnu/store/...f32r-udev.conf
EUDEV_RULES_DIRECTORY=/gnu/store/...cx44-udev-rules/lib/udev/rules.d
--8<---------------cut here---------------end--------------->8---

trick somewhere to help users find the rules directory.
-- 
Pierre Neidhardt