Today I gave it one last try, by zipping /boot/system/develop on the 32-Bit machine.
Then, on the 64-Bit machine, I unzipped this whole tree to /boot/home/x86/boot/system/develop.
Finally, the cross-compiler will work correctly with:
/boot/home/Desktop/haiku/generated/cross-tools-x86/bin/i586-pc-haiku-gcc \
--sysroot=/boot/home/x86 <your arguments here>
…but it seems “hacky” and unnecessarily cumbersome to me 