Hmm I keep pondering ways to open up NPCs on slaves without clicking in windows.

I think I have a fairly esoteric way to actually do it if one thing is possible. I've never tried this, so I could be full of beans.

Can you open up an NPC by targeting them and then right clicking on their portrait? Similar to one method of opening a trade window with a PC, but with an NPC?

If you can do that I think you can do one of a number of ways to open up the window on the slave window, whether you trigger a macro that will assist the main to target the NPC and than emulate a right click at that location on the screen. The portrait of your target does not move around like the location of the NPC in the slave windows does. Another potential way to do it is some variant of mouse broadcasting for just that click, hover your cursor on the main screen and hit a macro or hold down another key to turn on mouse broadcasting, right click on main window, etc.

There's got to be a way to do it, between the mouseover functionality that can be used for addons like Grid or maybe if you try /trade with the NPC. I thought there would be an /interact slash command but there isn't, and /use is the command for gear items in the screen, anyone tried to /use NPC with a macro?