What I do with leftover udon noodles.... I pack them up in a tupperware
with any toppings separate (all precooked since they are leftovers!).
Also, I pack the udon soup separate (a jam jar seals quite well).
At work I have a microwave but what works even better is the hot water
dispenser. Pour hot water in the noodles tupperware, wait a few minutes
(or the time to walk to a sink) and drain. Heat the soup & toppings in
the microwave. Then pour onto drained noodles.
Regarding heating miso, it's supposed to change the chemical composition to
boil miso, so be careful when reheating. Actually cold miso soup is quite
refreshing, so I don't usually reheat leftovers. It's very easy to mix
up a small batch of miso soup so I've never tried to freeze it, so don't
know if that will change how it tastes.