There are several ways in which this could be approached, but the easiest way for your purposes and the mostly widely supported at this time, would be to a) create mirrored glyphs in the font and b) use the 'calt' layout feature to contextually substitute them when not followed by another letter glyph.

Would the .fina feature be totally inappropriate for this?

(Entirely possible, because I recall some threads where attempts were made to make .init, .isol, .medi and .fina work with Latin fonts, and some software doesn't properly handle these four for non-Arabic glyphs. Well, that's what I recall, at least.)