Does anyone know if a tablet computer could be used as an LCD monitor on a 5D? The reason I ask is that I can't find any comprehensive information and I want to kill two bird with one stone...

If so, I would like recommendations as to which one will work best, but also small.

I doubt it. You'd be better off buying a cheap portable LCD TV with a video in. That's what I did at first, although I've since graduated to a SmallHD DP6.

@Tim, you are aware of the aspect ratio and resolution switches that happen on the 5DM2 HDMI output right?

Monitors that are 5DM2 aware like the Small and Zacuto have firmware to handle it and maximize it's usefulness despite Canon's design flaws. Whatever tablet you try to hook up will not only need a video input port but software that handles it.

Composite video is probably better behaved but so low resolution .....

Here's one of several threads that discuss the issues:
http://www.dvinfo.net/forum/canon-eos-5d-mk-ii-hd/502705-5d-mk-ii-hdmi-video-out-video-recorders.html

Just like you, I considered virtually every option - including the idea of using my iPad 2 but after much research, I opted for the SmallHD DP6.
5.6" Small HD LCD Monitor, On Camera Field Monitor - the DP6 by SmallHD (http://www.smallhd.com/Products/DP6.html)
I'm so pleased I did, the quality is stunning and the software copes with the 5D Mk2 quirks admirably.
The recent firmware update has improved the Focus Assist still further and now I'd confidently say that if it's out of focus, it's down to operator error - the DP6 really is that good.

There is this:

DSLR Controller :: Home (http://dslrcontroller.com/)

Some people use it for video too..

DSLR controller
A mate of mine has been controlling his EOS (7D) with this on his ACer Iconia tablet

DSLR Controller :: Home

And says it works great.
__________________
Avey
theactionhouse.com

I have the Zacuto EVF. Love it. And I can loop through it with an HDMI out to a larger client monitor if I want. If there's a reason for you to use a tablet instead of a monitor, I think Toshiba and possibly a few others make them with HDMI inputs.