I see one way to do it "the simple way" (but not elegant) : enter the activity in real work. It will be a good solution for "emergency operations", when we need to exceptionnally plan 2 or 3 days the next week for helping on a project.
I agree with you.
Not elegant but very efficient.
To use for few days tasks.
The "enforce planning" plannig mode is a way to solve your use case, but as you mention, not the one I would agree with.
Because this means any project leader may "steal" resources to other project leaders.
IMO, I would prefer a planning mode that works the other way round : the Project Leader using the resource uses some planning mode "can free resource", that will allow other project leaders to use resource even if buy on this task.
Other option is the "automatic planning calculation" feature, included in the roadmap (but not planned for tommorow

)
This would include 2 automatic planning modes :
=> plan what has to be planned : every project that has been chaged will be included in the global planning (this should be planned for instance once per hour)
=> plan all : every non closed project will be planned, all together (this should be planned for instance every night)
The second planning would instantly solve your use case as long as you correctly define priority for projects.
Of course, this should be parametered by admin (enabled on not) and with possibility to exclude some project from global planning.