I think the only way to get the keyboard input is mentioned in that Cockos forum post:
Our plugins check the “Plugin editor requires keyboard focus” option in the Projucer, but still Reaper won’t give us focus by default.
The same applies to Ardour/Mixbus, there is a specific button to toggle whether or not a plugin is explicitly allowed focus or not. This stops plugins from implicitly capturing things like transport shortcuts, but it can be annoying from the plugin dev side as well 