Interesting. What is the reason for grouping method calls together? Is this to reduce the number of requests sent to the server? If not, is there no way of providing a higher level operation instead of bundled discrete operations?

In any case, I think you could have something a lot closer to REST, even if you carry on with grouping calls together.

Something like:

    [
        [ "GET", "/messages", { "search": "foo" }],
        [ "GET", "/mailboxes", { "etag": "bar" }]
    ]

Obviously, you're not limited to HTTP verbs if you do it like this, but it's a familiar metaphor.