I don't know how much help I'll be as my experience with this harness has been limited to use with Mr. Limpy... but hopefully it'll be of some use!
I've not had any issues with my packer falling out. The only thing is that sometimes the top of the packer (like, the end of the shaft that sits closest to your body if that makes sense) sometimes pokes forward and comes out of the hole. It's not excessive though and, like I said, the packer has never actually fallen out. I think it probably only comes out because I do a lot of walking — it gets jiggled about a bit I guess!
I do find that the packer sits just a bit lower than I'd like, but assuming you're not going to be running around in a speedo I don't think it's too much of an issue. Once you've got your pants on it looks pretty normal, and to be honest, it'd be far weirder for anybody to be examining your crotch that closely!
I've tried a LOT of harnesses and this is the only one I've had any marginal amount of success with. As you said, the full monty style tend to decapitate packers or stop them from sitting flat in your undies. This one just lets it hang quite normally, though that being said I would recommend tighter underwear to keep it sitting that way. Though having said that you've mentioned you'll be getting the Reelmagik which doesn't seem to stand to attention like Mr Limpy.
Looking at the Reelmagik the shaft looks narrower than the Limpy which might be a bit loose in the tool belt harness, but having said that the back looks pretty wide which should stop it coming out. As you mentioned the only real issue I can imagine is the balls hanging a bit low. If you're wearing boxer briefs or briefs you could probably just hoik the harness up a bit higher and your underpants should keep it nicely in place.
Overall the harness is actually very good in my opinion. It's comfortable (tend to forget I'm wearing it) and the leg straps stop it from swinging and twisting all over the place. It's kept my packer secure so far despite lots of physical exertion and it's relatively inexpensive — I'd definitely give it a go!
