In addition to the regular distcc handbook, you will need the distcc crosscompilation guide.
The thing is that, all the involved hosts must have a valid toolchain for the relevant architecture. That is easy to do in gentoo. For example, if you have a 32 bits machine and you want to compile for amd64 on it, you can probably generate a toolchain for it by emerging crossdev, and running "crossdev -pv x86_64-pc-linux-gnu" to build that toolchain, then just setup distcc to use it.
In fedora, surely there's a way to properly install and setup foreign toolchains, else you will have to do it yourself, which might be a bit complicated in some situations. Having different gcc and glibc versions never helps, certainly, but note that the gcc version that really matters to distcc is the one you'll be using for the target, which is not necessarily the one that your system brings by default.
I can't help you with fedora, because I know nothing about it.
One alternative could be to compile the given toolchain into gentoo, the quickpkg it, and then take it to your fedora machine. This might or might not work depending on a number of factors.