Been looking into this deeper.  In my attempts I wasn't able to modify the z-index of the dialog window in a modal scenario, with out some script.  The z-index that you set in CSS only applies for a non-modal scenario it looks like.  I'm not sure if there's a bug here or if that's the intended behavior, but I've fired off a couple of emails to verify.  Either way, here's a bit of trick javscript to force the dialog window to use a z-index that you want.  It replaces one of the WebDialogWindow private functions which is generally a no-no, but until I find a better way to do this through the public API, the script below will have to suffice.  I added mine to the pageLoad client-side function, so that it will get executed during the pageLoad event (fired by AJAX).  The only thing you really need to modify is the line where you see zIndex=100.  In the script included with the product this is 99999.

function pageLoad(args){ 

$IG.DialogMoveBehavior.prototype._doZ=
function(lid, elem, lidZ)
{
    this._newZ = this._oldZ = $util.getStyleValue(null, 'zIndex', elem);
    if(!lidZ)
        lidZ = $util.toInt($util.getStyleValue(null, 'zIndex', lid), 0);
    if(lidZ < 10)
        lid.style.zIndex = lidZ = 100; //z-index will default to 100 in a modal scenario
    if($util.toInt(this._oldZ, 0) <= lidZ)
        elem.style.zIndex = this._newZ = lidZ + 1;
    return lidZ;
};

}

Check the z-index of the dialog (and the dropdown or menu).  The dialog needs to display 'on top' of everything, so it's given an enormously high default z-index.  The dropdown portion of any non native dropdown control is an absolutely positioned container which also must get displayed above everything else.  It sounds like the Dialog Window is set to a z-index which is higher than the dropdown portion of the webcombo and webdatechooser (which I think use 1000).  Choosing which control ultimately displays on top is a matter of setting the z-index. The reason the classic HTML Select 'works' is because it's a native control to the browser which is actually a separate window.  Ever try to get an HTML element to appear over top of the HTML Select.. ?  Not fun.  virtually impossible without a buch of messy hacks. 

Anyway, to remedy your scenario you'll want to set the DialogWindow's z-index to a value lower than the dropdown, or set the dropdown's z-index higher than the DialogWindow.  Both can be set using the controls css class, and the z-index property.  I'll check with the product team and see if there's a reason we can't default this to something that will work better with other controls without forcing you to manually adjust the z-index. 

 -Tony