This ship is not really my period, but I'll try and help where I can.
These are two postcards I have of her produced by 2 different companies but both show the same view of her docking at Barrow:
When this kit was first released some kind gentlement sent me several hundred photos on CD of the present Mikasa.
I forget who it was. I'm happy sharing them with you, but I wish to have his permission first, perhaps he will see this and come forward?
Finally to answer your question about lower hull plating.
There's is a fairly good photo of Mikasa in dry-dock in Japanese Naval Vessels Illustrated 1869-1945 Vol1: Battleships by Fukui
This book is too precious to scan from, but on p50 we can see her lower hull whilst in dry-dock from the port bow perspective.
There are many wooden beams in the way, but it would appear any straking is not too pronouced, perhaps because the anti-fouling paint gives a good smooth coverage, but you can see some light lines below the waterline.
In terms of plans I cannot think of any decent ones for this ship off the top of my head, but there is likely some in the wealth of Japanese publications for modellers which have been around since the 1970s.
Cheers
LB