I have been told this is possible, but I have not figured out how. May not be possible on my tablet. __________________________________
CORNBLOSSOM

Hi Barry

PD13 should be able to project onto a portable screen as long as Windows sees it as an available monitor. HDMI is one way to connect, but that's almost certainly going to be an output on any tablet that has one (as opposed to a dedicated monitor, which will always have an input connector.

If the device has WiDi, then it will connect at full speed over WiFi to your main computer. there are other ways of connecting, but this is a good place to start

I'll see if I can connect my original Surface Pro to my desktop as a 3rd display and let you know if PD13 can project to it. Stay tuned...

YouTube/optodata

DS365 | Win11 Pro | Ryzen 9 3950X | RTX 4070 Ti | 32GB RAM | 10TB SSDs | 5K+4K HDR monitors

Canon Vixia GX10 (4K 60p) | HF G30 (HD 60p) | Yi Action+ 4K | 360Fly 4K 360°

There were a few Andriod and Apple apps I ran across, but I've got a Windows Stream 8. HP Envy Phoenix/4thGen i7-4770(4@3.4GHz~turbo>3.9)
Nvidia GTX 960(4GB)/16GB DDR3/
Canon Vixia HV30/HF-M40/HF-M41/HF-G20/Olympus E-PL5.
Tape capture using 6 VCR, TBC-1000, Elite BVP4+, Sony D8 camcorder with TBC.
https://www.facebook.com/BarryAFTT

Those might be worth checking out. I've just found that it's easy to connect a Surface to an external display, but it looks like the computer always wants to be the source of the display and not receive and display the signal from another source.

You could use an app like TeamViewer on the portable device to wirelessly duplicate an existing monitor, but remote access apps like that are optimized for interaction and won't necessarily give you full resolution in real-time.

There's a 7" 1366x768 monitor on Amazon for $120, or another 7" one that supports up to 1920x1444 for $180. Those are in the ballpark of the cheapest tablets with HD resolution, and they're designed to be just USB or HDMI monitors so there wouldn't be any extra work to get them connected and working right away.

Just my $0.02

YouTube/optodata

DS365 | Win11 Pro | Ryzen 9 3950X | RTX 4070 Ti | 32GB RAM | 10TB SSDs | 5K+4K HDR monitors

Canon Vixia GX10 (4K 60p) | HF G30 (HD 60p) | Yi Action+ 4K | 360Fly 4K 360°

The only way you can do that today with a regular tablet is from tablet to other displays, not reverse.
It's the same as laptops displays - they can only be used as output from the device itself, don't have an input from outside.

The only tablet that can be used in "reverse" mode is nVidia Shield Tablet that can use the streaming capability of the Kepler or Maxwell video cards installed in a PC, to display the game running on PC on the tablet. It requires a connection to a 5GHz router.
And... I have no idea if it can be used to stream something else like PD13 output.
http://www.geforce.com//whats-new/guides/nvidia-shield-portable-user-guide#4

The easy way is... to buy a monitor

Optodata,
I saw both of those items before I posted. The little monitor won't do it for me, as I need to hang it from under a cabinet.
The other, the field monitor has a native resolution of 800x480, I have that now and it's not quite sharp enough. I'll need to spring for a higher priced field monitor.
Those slick shiny screens on tablets would be perfect. Sigh. HP Envy Phoenix/4thGen i7-4770(4@3.4GHz~turbo>3.9)
Nvidia GTX 960(4GB)/16GB DDR3/
Canon Vixia HV30/HF-M40/HF-M41/HF-G20/Olympus E-PL5.
Tape capture using 6 VCR, TBC-1000, Elite BVP4+, Sony D8 camcorder with TBC.
https://www.facebook.com/BarryAFTT

Barry, you mentioned a couple of Android and iOS apps you came across. I did a quick search for "tablet as extra screen" and found this and this.

Those articles are a couple years old but the apps they describe certainly seem to fit the bill, especially the iDisplay one. Have you had a chance to try either of them out? Either would apparently let you have your cake/(tablet) and eat/(project to) it too...

YouTube/optodata

DS365 | Win11 Pro | Ryzen 9 3950X | RTX 4070 Ti | 32GB RAM | 10TB SSDs | 5K+4K HDR monitors

Canon Vixia GX10 (4K 60p) | HF G30 (HD 60p) | Yi Action+ 4K | 360Fly 4K 360°