The ufs kernel module in linux supports many different ufs types :-) 44bsd, ufs2, sun, sunx86, hp, nextstep, nextstep-cd, openstep.
read-write is supported on 44bsd, ufs2, sun, and sunx86, if the module is compiled with rw support. Still not sure I would trust it read-write, though :-)
I am able to mount my freebsd slices under Slackware with:
mount -t ufs -o ro,ufstype=ufs2 /dev/sda1 /mnt
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