Maybe you can still working on INF, without an exact release to be expected, like sort of preparing some content for the future, when you find an engine you do not start from 0,0. Some models, textures, 2D stuff and sounds can be prepared.
You can work on it 3 times in a month when bored.
You don't need an engine to make models, sounds, textures and so on, all those will be just optimized for the engine when you got one, but generally you need the engine for coding and mapping. Everything that is not coding/engine related can be done and when you have an engine you have a much faster start.
You probably allready did game content, but since the staff has time now (no deadlines) they can make the best models, animations and textures as possible.