I'd like to say that in theory you can get away with this. If you're playing pieces with unison baritone/tuba lines and your baritones can play and project you may be able to survive. Changing that Bass Clarinet player to a Contra and having someone on Bari Sax would also help.

If you play everything correctly with a group like that, good balance/blend, right notes, dynamic contrast, etc. it is certainly possible. Just know that there are judges out there who you've taken a hit from as soon as they realize you don't have a tuba sound... (Not right, but you have in some minds)

As someone said below, you might try someone else to tuba. Flute players have similar facial features to good tuba players and might be successful. I know a lot of people who have been successful with a flute to tuba move!