I don't think you really need a pattern as all a show halter is is a noseband and a crownpiece and something to connect them in the middle. What did you have in mind?
I used to make brow bands using gimp. I made all kinds of patterns (checkers, stripes ect.) I think they would look fancy as a nose band on a show halter. I am wondering where you would get the supplies (rings, cable, buckles and riveters and also the strapping. Any ideas other ideas I welcome. I am looking for a fun project.
You can buy the crownpieces separately from the nosebands so you could simply buy the completed headstall of your choice and then add the noseband for a custom look if you wanted. That's only if you don't want to mess with making the rest of it!
I love doing crafty stuff so can't wait to see what you come up with. What is gimp though? I thought I was familiar with most materials after working in a craft store for three years but I haven't heard of that one.