Hello Simon,
I had a browse through the issues that have come in, and no-one has asked that before, so you may be ahead of the field here. (Either that, or whoever has modifed this before found the process so trivial that they didn't bother to post about it. Take your pick. )
I'd say "Just try it." I can't see any reason why expanding that table by a few rows would present a problem (Apart from possibly at upgrade time if the default data there changes.) I don't see why it should be used by anything else.
Have a snuffle around, check out bop_sinfo for SREL . . . but just go on a test system and try. Should be fine.
(Famous last words? I hope not!)
Having said that, there was this site which noted weird and wonderful side effects of working with this particular field, when they were working with a specific scenario of manually changing the Tenant of the Contact who worked on the Knowledge Document. The problems arose more from the unexpected manual change of the Tenant than the field.
But if your workflow involved anything like that, then testing around these areas would be recommended.
Thanks, Kyle_R.