A few times I developed Fomapan 100 using Rodinal in my fridge, which I keep at 6 °C. The negs came out fine (I checked them now, for the sake of this post), with good density. The grain didn't become noticably much smaller, this is why I abandoned the idea, which was actually rather good, since there was no need to worry about controlling the temperature of the developing tank with the help of a water bath. I calculated the development time using an approximated thermal coefficient. Thanks Neelin for giving us the exact figure.
According to my notes, I used Rodinal at 1+25 and agitated normally.
Have fun with it!