If a magus uses Rune Magic to emulate the effect of an originally instantaneous Creo Corus/Mentem to increase an attribute (up to +5), what happens?
Will the increase last "only" for as long as the rune is inscribed?
Presumably, yes.
Would probably also cause warping.
Well, that's certainly how the vitkir would have to do it so it makes sense for hermetic rune magic too. Of course, one trick that a vitkar might use that would be of interest to a hermetic magus is to incorporate an AC into the effect and keep it someplace safey. The effect would still cause Warping, but the Inscription is unlikely to ever be damaged and so only a single casting is necessary.