I think that would be hard to do.
I suggest you make the soup with whatever you want in it and then thaw it
in the microwave at work.
If you use udon Japanese noodles undercook the them a tad and let it cook
up really hot at work.
For Rice, just add it in it'll be fine.
Put it in a container that won't leak.
You could also bring this to work unfrozen and I think it would be fine.
Adding the miso later would be troublesome.
You can also make Japanese rice soup, add dashi (which is fish boulon) add
any veggies you want fish, chicken, tofu I put egg whites in mine and boil
it up. Put it in your container, at the end add a bit of "ponsu" or soy
You can freeze this or just bring it to work as is.
At work top it with Japanese nori flakes.
How about making Japanese rice balls ( onegiri) to go with your soup
Put a bit of saltwater on your hands, take some cooled down rice about a
handful and sqeeze it in you hands forming a triangle. When it's all
compact and nice you can stick something in the middle of it, for example
tuna, or eggs, umeboshi etc. anything really.....
Then form the triangle again in a piece of saranwrap, and take it with
you. When your at work wrap it in nori.
You can also mix the rice with something before...you make the balls
(triangles really). I like mixing rice with ground seaseme seeds.
Put some beans in the rice when you cook it and then make the rice balls
later. Any beans are fine. Japanese like peas.
Asuki beans are great then put blace seseame seeds on it. Tastes great.....
Hope this has helped you.......
PS I would'nt use Konbu again all the flavor comes out when boiling and
it's not so nice anymore.....