Cold stuffed mushrooms?
In general, cold food needs stronger flavors to taste equally good.  A little more seasoning, a little stronger spicing, a touch more acidity.

However, you also have to consider texture and cultural expectations.

I am not sure that cold cooked mushrooms are... expected.  You might want to try it in advance and see what you think.  On the other hand, raw mushrooms would be dissonant as well with stuffing.

I would suggest reassessing what you are taking.  Perhaps a crudite, where your fillings transform into dips, and you supplement the (raw) mushrooms with other vegetables might be a logistically more appropriate dish.  

If you want hot dips, they can probably be done in a microwave, if your office has one.