My first answer was only on the medical aspect of it.
On Kilimanjaro there's none of what mountaineers call "technical climbing, so I think that the life of other climbers are not really at stake.
But of course it is desirable that your and your climbing buddies aerobic power are not too far apart. Caregiver raises the ethical aspect. I do not know how these climbs are organized but if the whole party can not continue because of one person, that would be sad.
Since I take a betablocker, my aerobic power has not suffered much. It is my peak power that has suffered.
My answer would be "no". The beta blocker limits your cardiac output. I have climbed mountains in the past and a situation may arise when the life of another climber may depend upon your ability to function fully - and you cannot do so when you have a drug limiting cardiac output. When climbing you have not only the responsibility for your own life and safety but of others in the party. It is not ethical to place yourself in a situation where you cannot pull your own weight.
Sounds like a wonderful adventure. If you are a runner, you can try jogging with the beta blocker to get an idea about how your heart adjusts to increased oxygen demand. I don't know if your rhythm issue is AF or not. If it is, there is a blog for hikers and runners with AF and they talk about high altitude climbs. See afibrunners blog.
The betablocker will limit your maximum heart rate. Therefore you will perhaps not be able to climb at a speed you used to be able to before you had the betablocker.
If you keep that in mind and that is no problem for you (and your climbing buddies) I see no reason why you should not climb the Kilimanjaro.
I wish you a great climb!