I've seen an osteopath for work on my scull plates without problems, but I was lucky in that he was very experienced, and happened to have done his thesis on sports related concussions. It's typically called craniosacral therapy. I would just be cautious with the message therapist, as Mark said it shouldn't hurt, and they shouldn't work with too much pressure, which I think most message therapist tend to do so. Hopefully he'll be working on you from his osteopath capacity. With a lot of these alternative therapies, it really depends on the individual therapist.
My aunt also did some work on me, she's a physio with lots of extra training. She said she felt like like my scull was 'stuck' and had lots of 'blocked energy', so it's just very gentle and slow movements, and into the jaw and neck as well. It might help as you have that specific spot at the top of your head. The scull isn't fused into one solid piece, so it should have some movement in it.
It only helped me a tiny bit with my headaches, but it did help with my neck and upper back pain.