I tried just now on my machines: An intel core 2 duo equipped with win vista x64, using Qt-4.3.3 compiled with MSVC 2008 express (x86), and an UltraSparc5, using linux (but userland 32bit), using Qt-4.3.3 compiled with gcc-4.1.2.
A .rcc file created on linux that contains a txt file and a png image works flawlessy on windows. I managed to load .rcc, and display text and image.
A .rcc file created on windows that contains an utf8 txt file works on linux. I guess should work also with not unicode txt files, but I didn't get to display non ascii characters that way.
I can't show images on sparc, because I got no X server on it.

It looks like that .rcc files are platform independent (at least when contains unicode text).