Well I have a few possible ideas:

An Octopus tattoo would fit into what you have.
A few large kanji poems for backs.
Large Japanese Clan symbols.
Maybe some Chinese wood block prints.

Maybe blue body paint similar to what was used in the British isles.
https://imgur.com/FtdewKy

I don't know if this could be done, but tribal body scarring might fit in well in Kenshi. I don't think it actually needs to be raised up, just shaded and highlighted to give it depth. Just make a few small scars and then do a lot of copy/paste to make a pattern.

Thanks tommy, as always your ideas at tops. Woad patterns and scars would be good for variety

put a couple of 'woad' designs in for the moment. Tried scars but will have to play with it more to make it look good.