The sidebar on page 116 of Complete Adventurer (covers weapon similarities in regards to chosen feats) does not mention mercurial weapons. There may be another note covering these somewhere else though, but I'm not aware of it.
Personally, I would not allow longsword feats to apply to a mercurial longsword, but I would have no problem with someone wanting to retrain their existing longsword feats into mercurial longsword feats.