There was a plugin, years ago, called Parian that claimed to render Farsi text correctly. From what I can tell, it hasn't been updated in a very long time and there's not much chance it would work in any version later than Photoshop 7. I don't know of one that's current.
On the brighter side, there should be OpenType fonts able to form words correctly in Photoshop CS6 or later, with the expanded support for OpenType features. My own solution in a situation like this would be to shift over to InDesign, which has extensive support for position-sensitive alphabets, do the typography there, and copy/paste back into Photoshop. Illustrator may also support that feature, but InDesign is king of typesetting. In either case, you do have to start with an OpenType font that has the capability built in.
Longer term, you might visit the Adobe wishform page and request that as a feature. Lobbying for features does work.