The Original reason for a Kilt Pin is just a weight to hold your apron down.
Now it is an excuse for us guys to show a little sparkly.
So custom pins are the perfect way to do both.

Wearing a pin horizontal is perfectly acceptable. (there are no written rules) However remember that the lower apron edge moves quite a bit and the last thing you want is for your pin to get caught on something and harm either the pin or the kilt.

That said, a woodworkers plane would make a perfect pin horizontal or vertical. Can your design be made so it is instantly recognizable for what is supposed to be?

An alternate idea would be a chisel. 1. instantly recognized as the tool of a woodworker. (It is the preferred weapon of the woodworker is it not?) 2. Can be worn vertical. 3. would be easy to cast with minimum detail required to make it recognizable. 4. and heck, what is a plane anyway but a chisel mounted in a frame.

Jonesing for pics.