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.
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!