An hour of hacky tinering and it's running at the native res and aspect ratio, and the results are rather pretty I think:
(click for full size version)
I was surprised to find that only a few things were resolution-dependant. The HUD was the most obvious, since it used hard-coded positional values, so it now uses relative offsets from the screen edges. The menus are all borked too, since they're hardcoded to 800x600 so end up huddled in the bottom corner, I'll worry about them later.
The problem is that I think it breaks the gameplay - searching and exploring the map is a large part of the fun and challenge, and you don't get that when you can see half or a third of it on screen at a time. In fact it makes some maps laughably easy. But visually it looks so good I'm not sure I want to let it go.