A bit of confusion here over your use of the word 'your', so I am not sure who you are aiming this post at.
Just to be clear, Sling have nothing to do with Custom Remotes, that is all left to users like me. On the other hand the supported boxes page you refer to does belong to Sling. They won't answer your question so I will do it on their behalf. There are thousands of STBs available so to list them would be impractical. The only logical way to do it is via drop down menus. I agree with you that it's a bit Catch 22, in that you need to know the model before you can check, but I think that's inevitable.
As to you comment on the 'How to Create Custom Remotes', both Sling and I have pages like that so I don't know which one you are referring to ?