That works for a while, but eventually the donor plastics will also brittle up as well. I think over the next decade the vast majority that haven't already turned brittle will reach that point as well. Eventually there will be no more 5x0 series PowerBooks.
Looked into using white lithium grease. If it is good enough for the floppy drives, it should be acceptable for the hinges. HOWEVER, on auto parts sites, it seems like most manufactures have drifted away from using it, opting for a silicon based lubricant instead for parts around electrical components. The only thing I'm not clear on is if the silicon grease is an insulator or conductor. I think it is an insulator. That's good. If any excess were to drip I don't want any surprise shorts happening.
As to the plastic side of the equation, I'm wondering if building up the area around where the screw comes into the lid with an epoxy of some sort would help spread the load a little better. I'm thinking a 1 inch radius around where the screw goes in, and maybe 1.5-2 millimeters in thickness. Just a matter of which epoxy to use. Also, possibly using the hot air gun from my rework station set to something low like 200 degrees to preheat the plastic a bit to help it bond with the epoxy a little better. Obviously this whole paragraph would only be good for a lid whose plastics haven't yet broken.