Crossdomain Ajax call (POST)

I need a solution for an Ajax crossdomain action of type POST (not GET). Basically I want to provide users with a unique js script to add to their webpages like: <script src="[login to view URL]"></script> and in that script I need to provide the ability to call via ajax (or other means like swf or similar) a script on my server to post considerable ammount of content (that's why a POST is needed as opposite to a GET). I know and I already investigated solutions based on jsonp but unfortunally that solution seems to only support GET. My budget for the project is basically unknown, so bid above the project specified budget if you feel your solution worth it. Ask any questions you may have. Happy bidding.