The rivets in the boom ends are only to prevent them from falling out if the boom is not in use. Once the tension is on the rig they serve no purpose. The rig tension will keep the boom ends in place. Hence aluminium rivets are good enough for the job. Aluminium rivets are much easier to remove if the boom needs maintenance and as an added bonus there is no galvanic corrosion between an aluminium rivet and an aluminium boom.
Save your expensive Monel rivets for a rainy day when you have to attach something that does require the rivets to take some serious load.
BTW, I have used SS screws to lock the boom ends in place. I only need a screwdriver if the outhaul purchase jams or breaks. I always carry a screwdriver (part of my car jack kit and also on my Swiss army knife) but I seldom have a drill and a riveter with me at a sailing event.