Take a look at the Williams LRS (Low Reflex Sight). It is designed to sit pretty low for this purpose. I don't believe any milling is required to get it to co-witness the front sight.
Edit, I just realize you said XS big dot, I was thinking the ghost ring front. I'm not sure if this gets low enough.
Mossberg has a new pre-milled gun that can use RMR pattern sights. It may be cheaper/cleaner to get new guns and sell your current setup.