for the RISC processors, it should be relatively straightforward to do the 64-bit variant of ?[acl], given an existing 32-bit one, and i'd be surprised if the applications also didn't work immediately since they run in 64-bit mode on amd64. (anyway, they required relatively little work, and the compiler can help, because only long long and pointers are 64-bit, unlike gcc.)
the kernel work is another matter; the essential 64-bit changes have been made, but you still need to adapt or change the 32-bit MIPS kernel implementation to suit, and write drivers. what does the device use for graphics? i'm sure i've booted a 32-bit MIPS kernel on a potentially 64-bit MIPS processor. set a bit or two and it works; i don't think there was much more than that.
