Jorr,
Any duct or even grill will cause a pressure difference! It will always be best to leave the doors open. I'll make the 'grills' as large as possible. The hallway is bigger than any duct could ever be. With large return grills and decent air sealing of the house itself, <<1ACH @ 50 PA goal, The added heat loss, over a bare mini split, should be very small, but you are correct there will be some added loss due to infiltration. The added loss is, like the fan power, is a 'cost' of "distributing" the heat or cooling, that I'm going to pay.

Here is a ducted head for an inverter heat pump that will go down to -5F. Seems like a better idea than using a head that wasn't designed to be ducted.

http://www.fujitsugeneral.com/duct.htm

I agree with your idea of using a little bit of ducting vs putting a head in every room.

Jonr,
The head you linked to does indeed hide the ugly but it is still designed to serve a single room and has no provision or ability to handle 50 feet of duct or multiple ducts. I'll stick with the Panasonic fans and mini split in a closet. The closet collects ALL of the output of the mini split the fans divide it and send measured portions to the far corners of the house. Fwiw the 5 fans and control cost less than $1000 using internet prices.