Sorry to resuscitate this thread, but i simply wanted to point out the obvious for anyone with questions pertaining to special character removal with friendly alias.
I almost lost another day asking questions if I didn't search the forums and I hope this will help someone.
Opengeek's solution is further discussed here:
https://forums.modx.com/index.php/topic,52617.0.html. Translit is also available on Github:
http://github.com/opengeek/translit
In other words, just install translit using package manager and update the following keys in system settings:
friendly_alias_translit = noaccents
friendly_alias_translit_class = translit.modTransliterate
There are currently 3 tables: noaccents, german, russsian.
Once the settings are updated, simply create or update an existing resource with special characters in the alias to see if it woks.