I’m wondering if there is a way to force the enablement of a plugin freshly installed without restarting Eclipse?
It's possible using p2 APIs. However, not something I can remember now.
They're indeed outdated. What can make a plugin dynamic enough is mostly whether the extension point provider (the plugin that defines the extension point and reads the extension) does implement listening to change in the extension registry. Same thing for loading services from context or OSGi (although I believe OSGi does natively has dynamism for services).
I remember that in the past there was an “Apply changes” button after an installation, this is somehow what I try to restore for a specific installation.
Beware this was removed because in many cases, the result was incomplete or not satisfying: old extensions where remaining and failing, new ones ignored (because some plugins only look at extension registry once). Those are issues that would be relatively hard to avoid, so be careful.
Installation already done programmatically (feature well installed if I restart the rcp), I’m just missing the last rocket floor
I don't recall the exact API. I suggest you look at the history of the installation complete notification dialog, find a version when "Apply" was still here and then see what it was bound to.