I FOUND A SOLUTION TO BORDERS!
After trying all border removal apps on Aminet, only one really works.
OK, it does so after Workbench has started, as it is a WBStartup app, but it is working as far as I tested.
The app in question is this:
http://aminet.net/package/util/wb/2b_RemIBorder
Perhaps it helps Peter figure out how to add this into his library, but if it doesn't, at least we can all have borderless icons.
NOTE it doesn't remove icon filling though, so it's just for a plain background color workbench :/
I say it's better than nothing, for 3.X users.