I wanted to do the same thing you're doing, and I accomplished it with an addon domain. I made up some username and password that I don't use for the addon domain, and set the Document Root to: "/public_html". Now both my *.heliohost.org and my tld point to the same webspace.
Ok, so I think the recent downtime changed my account hosting package. Here is what cPanel says:
Everything used to be out of ∞, meaning inf. now it's 0 :-/. I can't get my addon domain to work, "ATTENTION: Using 1 out of 0 available Addon Domains".