Yeah Pete I can't imagine any circumstance with some fining and cold that the ale yeast would resume fermentation of the grapefruit. Agreed that grapefruit may not be the silver bullet though.I guess I was assuming it goes in the keg when racking from fermenter but I suppose if you open the keg up and add later or cold crash just before... But still, of all juices to reduce bitterness grapefruit seems weird. I would just see if the bitterness fades. I'm not completely sold no fermentation would happen. It's certainly not ideal conditions to pitch in but to me that means it will just be sluggish. I would definitely add juice after its in the keg a couple days, rather than right after racking when there is suspended yeast. Life wants to live...Acidity can mask some bitterness so I guess I understand where the grapefruit juice idea is going. I would try it out in a glass before committing to a full batch. The bitterness you're getting might not hide behind the acidity in the fruit juice. It could make the beer less palatable.I'm thinking it will ferment in the keg and dry out, so I don't think you will get the same profile as mixing it in a glass.
ale yeast and serving temps should be ok to add juice if it works..shouldn't/likely wouldn't ferment anymore.