Your right; it dulls the heads.
I sharpen my broadheads all the time; as they can dull from taking them in and out of the quiver; or from being pushed back in after shooting loosens them up a bit.
Having the middle of the blade dulled a little might increase performance a little. The simmons broadhead; that has a concave edge makes a big hole; but so does a zwickey sharpened all along the edge and then dulled in the middle. It pushes the skin a bit; and the sharp edge at the back of the head then makes a bigger hole. I tested this out on dead game; after seeing people sharpen and dull their zwickeys that way.
So; I wouldn't be too worried about the amount of dulling that occurs. A little lubricant on the edges can encourage the foam to grab the ferrule instead of the blade.
If you take a broadhead and heat it up and push it into the foam; that can help too.
I am in favor of sharp broadheads; and I am glad your concerned. There are lots of ways of carrying broadheads; the foam is a good and popular way to do it.
I would think that quivers with double clips on each arrow - and no hold on the broadhead probably keep the heads the sharpest.
you get an A+ for caring about it