I found a forum that has a high PR and is in my field. I'm not allowed to put my sitelink in my signature, but my URL is in my personal profile. Can I get any benefit from my posts that way?
I would say that if you write great posts people like me would not only look at your sig links but would also look at your profile to look at all your posts and website. You know to get a sense of who you are and your history to make sure your post was not a fluke and that you truly know what you are writing about. I would say the forum you want is smart for the fact they know who they are and that SEO work is vital to them. And that many will try and take advantage of them regardless if the users sig links are related to their forum or not. Did you read all their rules. Maybe after so many posts than you can place sig links. I know here at DP is something like 14 days or so. Maybe the forum has stricter rules by making you wait longer.
As far as you can index that profile in Google you will benefit from that link Even better if the link in the profile is DoFollow.