My solid bubble bar recipe has a consistency that could be easily molded - though not piped. Maybe you could mold it into a topper and then use M&P to adhere it to your cupcake base???
That's a little tricky when it's a wholesale lot of 75 to 100. Trying to come up with different ideas other than soap to to pipe on my cupcakes. I know I'm trying to come up with an idea that may not be possible But trying to think outside the box lol