Could it be possible for linode to allow us to compile and use our own kernel based on kernel sources we can choose freely (patched with the uml guest patchset obviously)?

Unfortunatly, we can not offer this due to security reasons. Allowing users to run their own kernel would allow users to run arbitrary code on the host directly. This would be a major risk to security for the other Linodes on the same host.

Ah, that's a way I hadn't looked at it; thanks for the quick reply and explanation.