A while ago, I mused about post- being a preposition-forming prefix. It now occurs to me that I fell into the trap of confounding the function and the category. I wrote, "Clearly though, the word post-Walkerton as used above is neither adjective, verb, nor noun." This is mistaken.
The context was How is the water post-Walkerton? Of course, post-Walkerton can be replaced with a prepositional phrase such as after Walkerton. It can also be replaced with an adverb such as recently. But what I overlooked was that it can also be replaced with a noun phrase such as these days or (with a slight adjustment to the verb tense) last year, or Monday. Given that, I think that post-Walkerton would make most sense as a noun, and that post- is not, in fact, a preposition-forming prefix.
So, it appears, sadly, that there is no productive way of creating new prepositions, though -wards is weakly productive (again, of intransitive prepositions, which many people wrongly call adverbs).
Incidentally, Geoff Pullum recently credited me with discovering post as a new preposition. This is true for certain senses of discover, but not for the sense that I was the first human to realise the situation. As commenters on Language Log have mentioned, the OED lists it. But the OED seems to miss a distinction between post and after, which commenter Mike Keesey explains nicely when he says, "'After' is neutral. 'Post' implies that the event/period in question has fundamentally changed the state of things."