The ID3v2.3 spec is 16 years old. TPE2 is universally mapped to Album Artist. Mapping to it anything else would more than likely mess up the metadata displayed by every audio player out there.

While not exactly standards there are some 'common usage' mappings used by various classical tag items. Yate v3.6 will more than likely be released on Monday. It contains a picker for common values in the Preferences-File List-Custom Mappings display. If you do not want to wait, the more or less common usage for Orchestra is:

custom field 'n' set in Preferences-File List-Custom Mappings
display name: Ensemble
ID3 mapping: TXXX/Orchestra
m4a mapping: Orchestra
FLAC mapping: ORCHESTRA

If you use the above settings for a custom field you will more than likely see any pre-existing orchestra values you may already have. Using Ensemble as opposed to Orchestra is strictly a matter of preference. All unmapped values can been seen on the UD Text panel. Once you assign the above mappings to a custom field, you can use the custom field on a custom panel. v3.6 also lets you directly map UDTIs to a custom panel.

There is a sample action called 'CONDUCTOR -> Conductor' on the Sample Actions page (accessible from the Help menu) which describes how to setup/import the six classical metadata items used by Audirvana. v3.6 will render the action/custom mappings somewhat moot as the built in picker does it for any custom field.

v3.6 also has canned settings for: Instrument, Period, Soloist, Style.

Yate currently maps 106 fields or field classes. You can define well defined mappings for ten fields as custom fields or a potentially infinite number of fields as User Defined Text Items (UDTIs).

I hope this helps 🙂