a question concerning voicing harmony and vowels

To begin, I feel rather silly at times doing this, as every time I mail the list
it's to ask it a question, as I hardly ever have anything to add to the
conversations going on.
Anyway, I was wondering.. I have a conlang in which there's voicing harmony with
adjacent consonants, that propagates to the left, so a series of affixes like
-s-v-ak collapses to -zvak.
On the other hand, there's also vowel assimilation that moves to the *right*. So,
ni+u~n becomes ni~n, with the nasalization carrying over to the i.
Is there a precedent for this? There's this feeling of nameless dread I have that
tells me this is *not* the way things are ordinarily done.

---
Shreyas