Bingo!
I replaced the .tga's of the other 3 'line'-tools (protractor, compass/circle and square/waypoints) with the line.tga image but renamed. And needed to replace the mouse cursor offset X,Y for the square image. Simple stuff really. My initial try was to change the 3 "Name" fields in the mouse.cfg file so it could share the line.tga file among the tools. But SH3 insists on those names or else uses default images.
Oh yeah, before I get into trouble: These files and all artistic rights belong to OLC Gui 1.2.6 or what/who/wherever he got them from.
Download this JSGME minimod from here
Once I figure out how to mess with layers and stuff in GIMP(so far been postponning/avoiding that) I intend to add N, NNE,ENE,E, etc. markings and their borders in between. Those angles are tough to find among all those tiny degrees.