N gauge cork for HO

I have, it works just fine for a low-profile roadbed. My experience is with Woodland Scenics foam rather than proper cork, but the width is about the same--it is just a smidge wider than HO scale track. If you're modeling a branch line or a bunch of spurs and want a way to simulate a minimal ballast profile, the N scale stuff is very handy.
 
Maybe you could turn it upside down and install it that way. It should be wide enough without the added spacer. Even if it wasn't, your rail beds would still be well supported without the added hassle of a 1/4" filler.