You should sing with them on and act like nobody is in there. Don't make eye contact and just keep singing while you wash (kind of eyes closed/rocking out) and dry your hands and then continue on with your day.
It will start a trend. As opposed to the sometimes awkward bathroom glances/conversations you have with co-workers with whom you never talk.
Normal conversation:
"Hey"
"Hey, have a good poop man. See you later"
New conversation:
"Hey"
"In the Ferrari or Jaguar, switchin four lanes. With the top down screamin out. Money ain't a thang"