Thanks guys. They are my own cables and housings, not Shimano supplied. Might try some auto grease. I've heard that can form an OK seal against water at the rear deraileur section. I am kinda tempted to use triflow, or maybe even some auto trans fluid.
Being slack, I tend to only remove cables to change them when shifting gets noticeably difficult, like when the plastic liner has almost rubbed through, or the cable has frayed or broken right at the front. Maybe I should try those MTB style cables with multiple seals and sheaths. Expensive tho, and hard to beat fresh ordinary cable for shift quality.