The only logic that theme_tinymce_theme() performs is “Force the 'simple' theme for some of the smaller textareas.” But my function only enables TinyMCE for 3 textareas, and none of them are in that list of “smaller textareas” so its totally unnecessary to call it.

But if anyone is thinking of using the technique I outlined, they should, of course, review the theme_tinymce_theme() code in the tinymce.module file.