If the percentage argument had any validity - and it doesn't! - then an aircraft flying at zero wing CL - i.e. approximately a zero 'g' bunt case - would be fantastically unstable longitudinally, because any change to the wing lift would be an INFINITELY LARGE PERCENTAGE CHANGE.
While a zero 'g' trim cannot be sustained indefinitely, due to the large and immovable object we all live on getting in the way eventually, it is perfectly possible to hold a zero 'g' point for quite some time - as anyone who's seen the footage taken from inside the various airliners that have been used for weightlessness experiments can attest. ('Vomit comet', I believe was one such vehicle)
Alternatively, shall we consider the impact of the percentage of lift argument on the directional axis. At zero sideslip, the sideforce on the forward fguselage - which is destabilising - is zero. Any change in beta will cause a sideforce here; once again, an INFINITE percentage increase. I think it will be widely acknowledged that aircraft are not, in general, directionally unstable at zero sideslip.