I still have two Slingcatchers that Sling discontinued a few years ago. Sling never updated the firmware so they don't connect to the newer Sling Pro-HD, only to my older models.
Sling charges $29 to purchase Android software (and you need a separate program for Tablets vs phones). I doubt Sling wants to give up that revenue stream so I would not expect them to readily give Roku users a free access. If Sling does allow Roku to develop a channel, expect to pay $29.
I've used the Slingcatcher with my PRO HD with no issues.
And the Google TV and WDTV channels are, in fact, free.