WoTC is not going to dogpile anyone for using static defenses in an
OGL product, as long that mechanic is described in the writer's own words. Those aren't copyrightable game mechanics, and besides, they're more interested in companies that actually violate their copyrights and trademarks.
So, you wanna write a game where saving throws/defenses are static instead of rolled? Go for it. It's completely legal, and
WoTC won't care anyway.
Honestly, the people that I worry about a little bit are the ones designing electronic character generators and online virtual tabletop for
4e. We all know there are people out there doing this, and I have no idea what
WoTC's reaction to it will be in the next year or so. We'll see what happens.