And the argument that a JSON-RPC request has to be a POST is irrelevant because the LP request as defined by the Deepbit spec is not a JSON-RPC request just because its response looks like a JSON-RPC response.
A a protocol is mutual agreement on proper procedure, so yeah, you can pretty well do anything you want if people play along. But you should not call it HTTP or JSON-RPC then, if it merely happens to look similar.
Diablo chooses to follow the very well established HTTP standard and the somewhat established JSON-RPC standard, rather than the quite ad-hoc LP spec. This is a good thing, even if current pool practice also accommodates the JSON-RPC-ish GET LP, as you indicate:
Fortunately, it's easy enough for pools and proxies to support both by not caring if there is a POST body or not.