Ghostbreed: If I choose OpenGL as output in DOSBox, would QeffectsGL work the way it should?
It most definitely won't. OpenGL is only used for drawing the final 2D image by DOSBox, the games themselves do not use the OpenGL functions to render the 3D stuff so the only thing QeffectsGL would be able to do would be filtering the final image - and that's "too late" to do most of the magic that QeffectsGL seems to be capable of. Glow might work but the results are in my experience awful when it's applied to the whole image (interface included).
The only exception being the DOS games that support hardware acceleration (like Tomb Raider and Carmageddon, I think). But those require a custom build of DOSBox that uses a wrapper like nGlide. I guess there's a chance you could somehow make nGlide go through QeffectsGL or something but God knows whether that's gonna deliver the desired results (or work at all).
Edit: But dang, going through nGlide to QeffectsGL would be pretty awesome, it could potentially add some serious eye candy to otherwise visually humble Glide games. Will have to check that out.
Edit 2: Ignore the nGlide + QeffectsGL talk. Read further below why.