Consider that hyperjump mass-lock, instead of being a binary jump/no-jump state, could instead cause the hyperjump targetting system to lose accuracy, causing the system target reticule to swing around wildly like a gimballed weapon when chaff is deployed. The degree of oscillation would be dependent on the strength of the mass-lock. When close to a planet or a station, the range of oscillation would be 360 degrees! A jump would still be possible (with an override), but would likely result in a misjump with severe consequences. This would perhaps allow a lucky jump in extremis to a valid system even when masslocked.