[MCahoon]

A sketch would help. Would your chassis be essentially a pentagon with the omniwheel in the front and traction wheels on the other 4 corners chained like a skid steer system? Given that it meets the bumper rules and fits into the starting dimensions, that is probably legal. The single wheel would provide forward/back traction but be able to slide sideways when the robot turns. That part would probably work. Dependinng on how many motors and how much weight your team wants to apply to the drive base it might reduce the power applied to the other 4 wheels.

Powering that single wheel would probably need its own motor/gearbox so that when your robot is spinning on it's axis that wheel would not be turning, but would turn when powering forward/backward.
Other than the pointed front end for deflecting collisions, what other advantages do you see?