Glad that you like it. It is really easy and really good. It's become one of my "make on Sunday, take to work all week" meals.
In fact, I think I'll do it again this weekend.
I don't find that it seizes up much. It does get stiff, but that's mainly from the cheese. The more cheese you use, the stiffer it will be when it's cold, but when you microwave it, all of the pasta separates again.
The reason rice would seize up like mad is that it would be essentially a risotto, and with the amount of starch rice liberates (especially the medium and short grain rices), it's going to be very sticky and clumpy.