It's possible. The review of Workbench 3.1 on Aminet was conducted on a machine with 3.0 ROMs.
However, running Workbench 2.1 on 2.04 ROMs is the only "official" instance of running a different version of Workbench than the Kickstart. One could argue that 3.5 and 3.9 also fit into this category, but I think they're a little different.
