I've contacted all of Boston, Hartford, Providence and Manchester NH Approach on occasions when I am skirting their areas and worried that some oncoming power traffic might be doing the same. They are usually pretty cooperative if you sound like you know what you are doing (or as in my case are good at faking it). Occasionally they don't "get" the distinction between a motorglider and a pure glider and will give you an altitude hold instruction that you respond with a quick "unable - I'm a pure glider" and they will cheerfully move the other guy for you. (And they really do move them for you!)

It's gotten easier since the dedicated 1202 code as they usually don't give me a new code. It's helpful to monitor the frequency for a while before you call up to get a sense of the workload and give and take. If it's really busy and tense I don't bother them. Once they establish radar contact I usually tell them I will advise before leaving the frequency and when I need to leave, a crisp "request to terminate service" is all that's needed.
ROY