Agree with Aurora, convection distributes heat to the food in the oven faster than just radiation or conduction thru air circulation, but it seems to me soup would heat a lot faster on the stove.
There are many articles out there both claiming that microwaves at the most cause harmless degradation of some nutrients and others that state one might as well chug-a-lug carcinogens than nuke your food.
Was a chemist for many years and sorta feel the effects are probably not to be feared, but the science will probably not matter.
Sadly, perception can become reality for many and hunterlax, you have to accomodate your clients.
Don't know of any really fast and easy way to heat the soup shy of nuking it (an unfortunate term since no ionizing radiation is involved in the process).
Maybe some hot plates, heck I don't know.
Good luck and if you find a solution please let us know.