Don't steal.

I would second the idea of leaving a note with your contact info saying that you'd be interested in buying the bike.

At my school, they would tag abandoned looking bikes with big yellow tags that had a date that the bike/tag had to be removed by before the university would remove it for being abandoned.

I don't think that posting on CL is good idea for two reasons:
1) A person who leaves their bike unattended for six months is very likely not going to be looking at the bikes section of CL.
2) It's a free ad for potential thieves looking for an easy bike to steal. To be honest, I don't know how likely this scenario actually is, but I am just being paranoid.