Hmm...
My first though at "bypass oil filter" is, essentially, "why would I want to bypass-filter my oil, when I'm full-flow filtering now?"
I'd still like to know how they're defining "bypass" - I'm still thinking that it means "bypass the filter." While dirty oil is usually better than no oil, that's just the first blush.
Having looked at the filter, I don't see any reason why we CAN'T mount it to an XJ - it's a remote filter housing. Therefore, find a place to mount it and a way to plumb it in, and you're golden.
I'd still like to know how the thing is built (sorry, but if I'm getting full-flow filtration now, I'd like to keep it that way!) before I got it, but I can't see it being any more difficult to install than any other remote filter setup. Don't forget to adjust your oil change amounts to reflect the filter and the lines. Might be a good idea to stick in an oil cooler while you're about it.
5-90