Yeah Almost no reason not to have a back up iron sight (BUIS) on there, it takes up really no real-estate on the rail, generally people move their holographic or red dot sights forward on that rail anyway, leaving plenty of room for a BUIS.
Of course my first recommendation is my favorite and incidentally the most reasonably priced, would be the Magpul Back Up Sight , Magpul MBUS- $50ish bucks right now online, I saw some cheaper too.
Also Troy are "nicer' as they are metal and actually battle sights rather than the injection-modeled plastic like the Magpul, they are like $100-$200.
But as the above poster said, you really want to just spend the $50 bucks and put a back up iron sight (which folds flat when you aren't using it) ... You can even do something like, sight your EOTech in at say 25 yards or whatever, and sight in your iron sights at 100 yds, etc. Just in case.
And I just had to sell my Bushmaster in September because I had ran out of money, and of course 30 days later got a big check, but hey, it was my baby for a long time, I loved it.