I didn't try HP like you asked, but hopefully there will be some some useful info to extract from my experience.
Not long ago, I got curious about whipped soap and thought the best idea is to practice on a rebatch: simply melting a couple of old bars with added water (I tried this on few recipes with a hardness around 40-50).
Obviously, the melted soap was too thick to put through a piping bag. So I just thought to add a little more water. The water got it thinner so it could be easily whipped and then piped.
I am far from being a piping bag expert, so I had a lot of trouble getting the desired thickness. Sometimes I would add too much water and it got too thin. But mixing it some more got it hard again.
The result wasn't perfectly smooth (like butter cream), it was a bit rougher, maybe like a meringue that you beat really stiff. But ok for a first try.
By the way, I don't know how you would "glue" the frosting to the cupcake.