Can I call a filter on all Rich Text fields? I'm looking to dynamically replace the ℠ glyph with &lt;sup&gt;SM&lt;/sup&gt; in all rich text fields, since that glyph doesn't exist in the font we're using. I'd like to avoid having to call the |replace filter on every field in every template, and I can't seem to get the customAsciiCharMappings general config setting to work either (I'm not sure if I really understand what it does). Does Craft have something like a global replace function/filter?
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.