Excellent! Yes, I agree it would be cool if mothballed ships were seizable. Defense should be zero. If you can get next to it & board it's yours. But when reactivated it would have zero supplies, of course. No combat tricks of seizing mothballed ships & using them immediately!
Automatic decay if they don't have a space yard nearby to maintain them is also good. Ok, I'll compromise on one component (randomly chosen) per turn as the decay rate.
The vast majority of mothballed ships will be in a spaceyard sector anyway.
Now the question is: How do we get MM to do this along with all the other things we're requesting?