The bit marked "For AMD64/Linux:" is a FreeBSD 5.x binary for prime number calculation:
here
You need to install
misc/compat5x for it to run, but it does do something. I don't know where to get work units, but you might be able to muddle through.
Setting MAKE_JOBS_NUMBER=7 and compiling
editors/openoffice.org-3 from source does a pretty good job of spiking the CPU temperature & bogging things down.
You could also use a
tmpfs(5) or a swap-backed (malloc)
md(4) of 800M or so for
/usr/obj and buildworld in a loop (maybe with -j<something insane>).