A permanent fix to get the Steam overlay working AND run fullscreen at your native resolution!
I found a fix for Abe's Exoddus and figure I'd share to save you all the frustration.
I was having the normal but annoying problem of no Steam overlay and black borders.
This will fix it for good.
All you need to do is download DXGL. See here: http://www.williamfeely.info/wiki/DXGL#Downloads
When installed, select "stretch to screen" and that's it. Your game should now be at your native resolution, with no borders, and a working Steam overlay. I currently have it running at 1920x1080 with no issues, or "squished" textures. You can also add more filtering to enhance the graphics a bit.
For those looking for a more technical explanation on how this works, this tiny program takes legacy DirectX versions 1-7 and makes them run in OpenGL. Most modern GPUs have trouble with legacy DX versions (from what I understand), and Exoddus is a DX6 game.
Also, this app *should* fix start-up and in-game crashes some people were having, but I can not confirm as I was not having crashing issues. This app should theroretically work with many legacy games. I have confirmed this works with Abe's Oddysee with the native resolution part - I still can't get the overlay to work however. Lastly, this app has no performance decreases that I have noticed, unlike the DDhack mod. Game runs 100%. All this was performed on W7 Ultimate x64.