I don't know if this will be any help but it's always difficult to find somewhere at Carnival time. There is a fairly new hostal called Mellow Yellow, it's owned by an English guy,Terry. It's a backpacker haunt and when I was there I was without my bike but asked and I THINK he said he could do something. It's got a great atmos and is 200m from Copacabana beach. There are a load of rooms that have bunk beds in and at Carnival time they put a third bunk on top. They have a website, sorry, I don't have it but it should be easy enough to Google. Nothing definite but worth a try.