Erm not sure if this qualifies. Not so long ago I was tackled with having to read a device memory at a very high address in 32-bit linux process (kernel is 64). The 32-bit mmap is unfortunately limited to range of protected mode PAE so it just wouldnt reach that high. So! I wrote my own syscall in assembly that would switch to long mode first so I could use long registers and then I got my page and switched back :)
In retrospective not a big deal, but it made me really happy for the rest of the day when I saw that address in pmap :)