As discovered by poltawski while reviewing MDL-49329 (the comment), there is no way to cancel installation of a subplugin.

I ended up with with workshop evaluation method which could not be cancelled from installation

My response was

I see. Hmm. This is because core_plugin_manager::can_cancel_plugin_installation() returns false for all subplugins. I remember adding that because I was in another situation: I tried to install mod_hotpot which comes with many subplugins in its package. In that case, the subplugins coming as a part of the parent plugin must not be cancellable because the parent module can implicitly require them to work properly.

We would somehow need to recognize that the subplugin is being installed independently and not as a part of the parent plugin. Not sure at the moment how to do that reliably...

Well, we could at least cover the scenario that (1) the parent plugin is a standard one and (2) the subplugin is additional. In that case, it is reasonable to expect that the subplugin's installation can be safely cancelled.