@caver01 That's true, I am also investigating udev and xrender as I was told by a friend that a rule could probably be defined to handle that behavior. Indeed, since the portable screen will be connected internally (inside the box) to the raspberry pi, we can know for certain which HDMI port is associated to the screen and therefore differentiate it from the other one which would be assumed to be an external screen.
EDIT: Actually, I realize now that contrary to USB ports, HDMI splitters only duplicate the input display to two outputs in an identical way, so I am no longer sure udev understands it is communicating with two different displays.
@caver01@rbaker I am also investigating using a USB Power Bank as a UPS (which I read from over at http://raspi-ups.appspot.com/en/index.jsp). Some models have smart charge features and are able to serve while charging and feature two USB ports (which is useful to provide power to both the Raspberry Pi and the portable screen).
is pretty cheap (~20 bucks),
has a 16.75A capactity,
provides 5V outputs,
provides 3A max on each of its 2 ports,
I am guessing this could be used as a UPS.
Just as a side note I have seen several interesting projects providing UPS boards for the Raspberry Pi. But they seem focused on powering it through the GPIO (as far as I understand) and therefore do not seem very appropriate to me if I want to power a portable screen as well.
Another side note to say that I don't have any stakes at Amazon (and my links aren't sponsored), it just seems convenient to provide examples :)
EDIT: changed the example provided for a more suitable USB Power Bank
@batman I was looking into some LCDs too and I added most of those to my wish list while I was laying in bed the other night lol. But I discovered those 2k monitors and the capacitive touch monitors when I was making this post. Really good looking LCDs!