In 1.1, the program provided limited contextual menu support for just channels and the console. Further support was added via the Popups plugin, but this caused a limitation - other plugins had no access to this system.
There are two potential solutions, at least. One would
be an actual context menu system within the application, and plugins could respond to context clicks passed to an (x, y) co-ordinate within their window or pane, with a context response (e.g. kCMContextUserInChan for a ctrl-clink in the userlist). Some sort of system may already exist, seeing as you can already at least ctrl-click on the userlist and get a context menu come up, even if it doesn't offer anything useful.
An alternative option, though, could be to have the popups plugin provide an IPC service for other plugins to utilise.