Your think would be right so long as you can assume the user is using:
1. A color-aware browser
2. A calibrated Monitor
3. A wide gamut monitor
Since your destination is the web, you probably can't assume any of these let alone all three. If the color you are trying to reproduce really is outside the sRGB gamut, then it's going to be outside the gamut of many user's monitors as well. Sending them adobeRGB images won't change that.
Also you're not quite right about what happens when the browser is not color managed. If you send an Adobe RGB image to something that assumes it's sRGB it will not look the same as that image converted to sRGB. If you want to see what an AdobeRGB image will look like in a system that assumes it is sRGB, open in photoshop and assign sRGB. This will show what those RGB numbers look like when interpreted in the sRGB space.
Also, you can embed a profile in a png—you need to go through the regular save dialog, not save for web. I'm not sure what browsers support it (Safari and Chrome on the Mac do).
As far as Quark goes—I stopped using it at version 6 (or was it 5?) and switched to inDesign. One of the reasons was those horrible color previews.