Can I call a filter on all Rich Text fields?
customAsciiCharMappings won't help you here.

There is the "Find and Replace" tool available to admins (Settings->Tools) in the Control Panel, but its' a one-off thing and searches/replaces against all textual field types (Plain Text, Rich Text, etc. both inside and outside of Matrix fields).

Honestly, probably just create a small Twig macro that wraps |replace and use that wherever you're outputting Rich Text fields on the site.