I understand that you want to be able to get the new application for the iPad without having to pay again the $30. Unfortunately, the SlingPlayer for iPad is a completely new application designed for the iPad. It offers a maximum resolution of 640x480; SlingPlayer for iPhone tops out at 320x240 resolution when run on an iPad in Compatibility mode. So, in this case if you want to install the iPad application, it will be necessary to purchase it over the App store.
If you've purchased SlingPlayer for iPhone, and you're okay with the smaller resolution, you can use it in Compatibility Mode on the iPad.
You can confirm this at this article: