Here is a list of what you can do:
You'll need to POST this request in the following JSON format:
{ "email" : "user@example.com", "list" : "listshortname" }
Returned will be a JSON document with this format:
{ "status" : 0, "errors" : { "subscribed" : 1 } }
status will be set to, 1 if there are no problems. If there are problems status will be set to 0 and errors will have an associated array holding what problems there were.