I have Ghost Square Cross. Adding rear mirror to the left handle bar end. The cap seems permanent. Should I peel the seal and force this issue?
Do you mean that the grip is closed on the end? Or the bar itself?
Either way - or maybe both - I'd cut it. Do a neat job, especially on the grip. Some grips, like a pair of ErgoGrips that I have, really could not accommodate a bar-end mirror. I suspect, however, that most factory installed grips are easy enough to cut.
Also, most handlebars - flat or risers - are meant to be cut if needed. For the bar, just remove the plastic plug and toss it. You might have to insert a wood screw or something so you can attach vice grips and pull it out.
Seconding @TomIrvine but will add that it is most likely the grips and not the bars themselves.
If the grips are friction fit -- the classic rubber that "slides" on (more like forced on...) and stays in place through friction alone -- then you can absolutely cut the ends off as needed.
If, however, the grips are "lock-on" and they are secured in place with a small screw on a lock ring collar at the end(s), this style of grip usually has a plastic core inside. Depending on the design, it may or may not be designed to have a removable end plug. Even if it's a solid end, you could maybe try drilling a hole to accept a plug, but it can be tough to do cleanly. I wouldn't recommend cutting this type of grip shorter as it likely could compromise the integrity of the plastic core.
The best solution might just be to buy new grips that will accept a mirror. Thankfully grips are relatively cheap at $15-20.