Webrocket, you bring up some good points, but there are some additional things to consider.
The police have a job to do--and that is to investigate what happenned to Brian. They also have full access to the entire case file, if they are selective about who they polygraph, this would be due to evidence and information that everyone else is not privy to. Their 'selections' are not random.
IF Clint knows about some illicit drug activity that took place that night, that is NO REASON to avoid the polygraph. Most police officers are not stupid, they know that college kids experiment with drugs. AND frankly, Clint would not be imperiled by this admission, especially if he provided the information he has on the condition that the police take no action as a result of his self incrimination (related to drugs). These arranagements are made all the time; very common and perfunctory. CLint's lawyer could EASILY facilitate this.
Boohoo that Clint is inconvenienced by the polygraph, Brian's family has a right to know what happenned to him!! Consider if you will, the unethical nature of denying closure to Brian's family. Also, as we all know, police resources are very scarce. Clint's refusal to take the polygraph is hindering the investigation. If he really is innocent, he is doing himself and everyone else a grave disservice.
Rememember, Brian was abducted and is presumed dead. Is this not a serious situation? In dire circumstances such as these noble and extreme actions are required, with friends like Clint, who needs enemies??