The serum is transported by blood. Hair has no blood traveling to it, only the root. So, unless Bacon shaved his body clean, his hair wouldn't have become invisible, as wouldn't all of the fur on the gorilla.
Whenever Sebastian is invisible and interacting with the other characters, Kevin Bacon's voice would be broadcast throughout several speakers randomly hidden throughout the set. This was done to make the reaction more genuine, since the actors would honestly not know where his voice was going to come from next, just as the characters would not know where Sebastian was.