the giant elephant in the room that doesn't seem to have been discussed is that trying to validate that field names meet some strict criteria when loading schema.xml doesn't really address dynamic fields – the patch ensures that <dynamicField name="..."/> configurations have names which are validated, but i don't see anything that would considering the actually field names people use with those dynamic fields – ie: "*_i" might be a perfectly valid dynamicField at startup, but that startup validation isn't going to help me if i index a document containing the field name "$ - foo_i"
In general, i'm opposed to the idea of "locking down" what field names can be used across the board. My preference would be to let people us any field name their heart desires, but provide better documentation on what field name restrictions exist on which features and provide (ie: "using a field name in function requires that the field name match ValidatorX; using a field name in fl requires can only be used with field names conform to ValidatorX and ValidatorY; etc...").
If we want to provide automated "validation" of these things for people, then let's make it part of the LukeRequestHandler: for any field name returned by the LukeRequestHandler, let's include a warnings section advising them which validation rules that field name doesn't match, and what features depend on that validation rule – this info could then easily be exposed in the admin UI.
We could also provide an optional UpdateProcessor people could configure with a list of individual Validators which could reject any document containing a field that didn't match the validator (or optionally: translate the field name to something thta did conform) to help people enforce these even on dynamic fields.
So by default: any field is allowed, but if i create one with a funky name (either explicitly or as a result of loading data using a dynamicField) the admin UI starts warning me that feature XYZ won't work with fields A, B, C; and if i want to ensure feature D works will all of my fields i add an update processor to ensure it.