Well, each race's entity file contains this:
shieldData
shieldAbsorbGrowthPerDamage 0.001
shieldAbsorbDecayRate 0.0125
shieldAbsorbBaseMin 0.15
AbsorbBase is the starting mitigation, increases .1% per damage point, and decays by 1.25% per second.
In theory, you can change the BaseMin to your desired number (like .60), null the decay rate, and invert GrowthPerDamage so it decreases rather than increases. The engine should be doing simple math with these, and unless it's trying to store these values in unsigned ints it should work.
The only possible issue would be what happens if it hits 0. The engine provides for a mitigation ceiling (each entity has the max mitigation value), but it doesn't have anything for a floor. So who knows what might happen if it goes negative.
