Well I finally got around to implementing this. It works for 3D, and for 2D symbols, but it doesn’t seem to work for labels.
content_ = 'Text' IF GLOB_PREVIEW_MODE = 3 THEN content_ = 'Fave' TEXT2 0, 0, content_
…works for a drawing symbol subtype object, but fails when you convert it to a label. (The favorite preview reads ‘Text’.)