I know they where not allowed in other contest's but what about in the custom contest.

i believe that as long as the total tank volume is less than 12g, you should be ok, but i'd wait for a judgement from the contest gods on that one!

Yes, you can have one in the custom contest.

Wouldn't the false back wall suggest a sump/refugium and if it's custom you could go as much as 12 gallons of display and 22 gallons of total volume??? IE you could divide a 20 gallon in half with a false back wall?

Yup, technically so.

Thanks Chris, just wanted to clarify.