Thermador Convection Warming Drawer
This is not a good idea for two reasons:


140 F is at the very edge of the "danger zone" for holding food.   These warming drawers are unlikely to be very accurate, so you could possible end up incubating your food and allowing pathogens to grow, not cooking it.  This could promote food-borne illness.
140 F is probably not high enough to get a quality and palatable result. It won't thicken starch thickened sauces, which require about 180 F. It won't cook green beans very well at that temperature, leaving you a somewhat raw and nasty casserole.

For some context on this, this recipe from Sous Vide Supreme for sous-vide for green bean casserole uses 185 F.  The water in sous-vide transfers heat to the target food far more effectively than air does in an oven or warming drawer.


If you are short on oven space, buying or borrowing a counter top oven may be the better solution.