Your concept/idea is do-able and you're on the right track, I think.
Do some research on silk screening for the basic process.
My initial impression would be to start with a set of main templates. These could be thin plastic/lexan sheets that are available at the big Blue Box. You'll need one template for each color you plan to apply.
Each template will be cut to the overall shape of the megaphone, and for each color, sketch out every part that exists ONLY in that color.. white, yellow, black, etc., then using a razor, cut those details out of each one. This is called a "negative" template.
From that point, it's a matter of your purple base coat, let dry, then lay on a template, spray in appropriate color over the cut-out portions, let dry, etc. Spray lightly with multiple coats to avoid having the colors run past their boundary on the piece.
A couple potential issues... if you use a paint that's reactive to plastic (IOW, melts it), you'll have a problem with those templates.
You'll need to frequently clean off the wet paint from the template to keep it from leaking/running where it shouldn't be. (maybe just go slow and do batches of 10-25, let the paint dry on the template, then start again).
BUT... If they're going to be personalized (like in the example), that's a whole 'nother kettle of fish. Huge amount of template work, especially if it's two color like the example, and you've got 100 names to do. For that, I might check the yellow pages for a local graphics shop that does custom-cut vinyl, like boat graphics. Their computer controlled machines can create your "negative" stencils way faster and easier than you ever could yourself. It might cost a buck, but I think you'd make it up in time and frustration savings. You'd use those stencils the same way as the template idea above.
On further thought, that might be the way to go for the whole project. Get all your templates made there instead of making them yourself.
Offer to let him sponsor the team and hang a few signs advertising his biz in exchange for his work... easy breezy!
Hope that gave you some ideas... as simple as that pic looks, I can see the paint process is going to be a challenge in a "production" environment. That's a pretty big project you've hooked. Good luck!