Today I managed to discover the minimum QL of an Improve (type) Weapons MRR!
Trying to improve a QL200 Slank Chop with a QL17 MRR, I got the message that I had to have a MRR of at least QL20 in order to do the improvement. 10% of the weapon's QL or better, in other words.
One less mystery in the world. :)