Abbra:
MC26 is a while off still, but perhaps it will address what you need.
In the mean time, you should build an MC view, in your normal computer interface, that only shows the media you want shown. That should be quite easy if you have your "on the computer" files separated by drive or top level directory. Just one or two include statements should do it.
Make sure that view totally works. Then, when viewing from another MC client, only use that view. That should cover that part.
For Gizmo and JRemote go to where you've been setting up your view (Media network > advanced > customize....). In there DUPLICATE the definition of the view you developed above.
Using a playlist is goofy. Use VIEWS.
Finally for DLNA, you need yet another definition:
Tools > Options > media network > add or configure DLNA servers > (select your server definition here) > customize views
In there you should again DUPLICATE the definition of your master view that you created in the first step in the main MC interface.
Why are there 3 places to do this? Well, there are 4 actually, because Theater View has it's own views too. So why are there FOUR places to do this? I'm not sure. I'm sure it's kind of a pain. But that's how it works. If you want your views to have similar behaviors, you have to edit all of them to be the same. They don't get shared or updated.
Good luck.
Brian.