I'm not sure why I was so stupid on this and it took me so long to figure it out. Apparently FO4's engine will give pretty much every object the slick shiny wet effect when it rains IF it is in an 'Outside World-space' and it will remain DRY if it is in an 'INSIDE World-space'. Unfortunately for us ALL player built homes are 'outside World-space'!
Again I am not sure why it took me so long to work this out but I will leave this revelation here for those that may NOT have figured it out yet:
Q) How do i know what is considered an 'Inside World-space'?
A) If you pass through a loading screen to enter a dwelling than it is an INSIDE World-space
So apparently until the Engine Editor or a possible DLC/patch comes out the ONLY customizable places our PC can live that will remain DRY are:
Home Plate in Diamond City
&
......wait is that it???????????
Anymore?
Really?
That's it?
WTF?
And yeah I did just spend a crazy amount of time and effort fixing up Covenant with the Spring Cleaning mod and some Console Commands to set it up as a nice place for my PC and a few named settlers/companions.
So what will we need ultimately...some sort of scripting thing where we can designate a certain zone as 'inside" or is this just unfixable with this game engine?