> what issues would interfere with using FFS?

According to http://library.morph.zone/Filesystems, Fast File System 2 (FFS) seems not so bad. It should be the same implementation by Olaf 'olsen' Barthel as used in OS4, up to and including DOS\6 and DOS\7 modes (long filenames). Unfortunately, the Linux FFS implementation lacks DOS\6 and DOS\7 modes, so if Linux should be able to access the partition, file name length is restricted to 31 characters.

> are there any inherent issues with sharing an SFS partition with both MorphOS and Linux?

The most inherent issue is probably that Linux doesn't know SFS by default. There is a kernel module available, but I don't know if it can be made to work with current kernel versions.