I know there's no private messaging system, but I'm wondering if users with high reps get contacted often to be offered jobs / projects.
There is no definitive answer to your question.
There are two ways you might end up with jobs / projects requests in Stack Overflow. Note the use of Might.
If your profile gives some kind of information on how to contact you then, yes. This usually happens when you have been particularly very helpful to a particular user and the user realizes that he or she could hire you to get something done. There are times when you get projects requests from unknown users as well. High Reps do play a part but not much. Else everyone with high reps would start getting offers. ;)
In my 7 years of posting answers in 4 different forums, I have had my good share of Project Requests. In fact I still get at least 1 in a month. I don't recollect a single instance (except which came from unknown users or through my blog) where my reps played a part. Why, I remember getting an offer when I had just 2k in Stack Overflow!