Don't some people have problems with something in grapefruit?
From my understanding grapefruit changes the way certain medication is metabolized. It can unpredictably increase the potency of a medication for instance. Calcium channel blockers, blood thinners, statins, a few others. Using a small amount of zest will most likely not cause anyone harm who throws back a few. Still, probably best to put out a verbal warning before serving. I'm on blood pressure meds and can't have it, I live on a grapefruit orchard and life is funny that way, but I would still drink your beer and likely be fine. I couldn't have it every night though.
None of this has anything to do with stuff getting clogged in your keg. When discussion goes from keg clogging to artery clogging it is a testament to all of our ages.