I hope your portable DVD can give you what you need. Good luck.
Just tried connecting my PC to my portable DVD player and it didn't work. But there are too many sources of error to know exactly why.
The DVD Player has a 1/4" din plug for Composite video, and another 1/4" din for "s-video". I already own a "composite to 1/4 inch" cable for connecting a DTV tuner to the player, so I found/purchased a D-Sub (VGA) to Composite (and S-video) adapter cable online. I then booted my PC using my regular monitor and set it to 720x480 resolution, rebooted to make sure it stayed that way, and then shutdown and connected my new adapter cable.
But my videocard only has DVI ports, so I had to use a DVI-to-VGA adapter on the end. I then plugged the cable into my DVD Player using my Composite-to-Din cable.
Turning on the PC, I could not get the screen to sync, with white lines of "boot text" racing to the right. Hoping it was just a "resolution" issue, I waited for Windows to start, but I never got the blue login screen. Windows was probably reporting an error I couldn't see. I shutdown using the power button on my PC and reconnected my monitor.
Possible sources of error:
The DVI to VGA adapter not producing a true VGA signal.
The cheap VGA-to-Composite cable not producing a "true" Composite signal (maybe "s-video" would work better?)
(I considered that the DVD Player might actually be a resolution other than 720x480... maybe 704x480? But the startup BIOS text wouldn't care about that.)
The DVD Player itself might not handle certain signals.
I found actual "portable LCD monitors" on eBay for around $85, but I was really hoping to come up with a solution that didn't cost me any money (besides the $2 adapter). I might try the experiment again using s-video or a different portable device someday, but for now, it doesn't look like it'll work with my setup.