Are there any Conservative synagogues that daven Nusach Sfard?
As already mentioned, the "official" Conservative siddurim use a slight variant of nusach Ashkenaz (the differences are not that great).
However, I have been in some "Conservative-like" (though officially unaffiliated) congregations that use nusach Sefard. These are, as you suggest, descendants of Orthodox congregations that once used this nusach. The congregations I am aware of use the Birnbaum siddur or a related older siddur.
The conservative movement uses a variant of Nusach Ashkenaz with some small changes (in terms of the percentage of the text changed), to express some rather large philosophical differences from Orthodox Judaism.
However, there are some Conservative-affiliated Sephardic synagogues that are egalitarian, but use the Sephardic nusach. These synagogues are thankfully the exception among Sephardim, rather than the rule.