Can a cellular unit share its failover OOB WWAN with non-cellular units?

Yes, this advanced configuration is known in the data center as end-of-row cellular gateway. 

The attached document explores the topology in which a single cellular-equipped Opengear unit shares its cellular WWAN connection with one or more non-cellular Opengear units during a primary network failure.  The document covers pros and cons vs. per-unit (top-of-rack) cellular and provides detailed setup instructions.

The document's worked example is of a row of four high density data center cabinets, in which a cellular-equipped IM7200 Infrastructure Manager (e.g. IM7248-2-DAC-LR) is installed in the first cabinet and CM7100 console servers are installed in the other three cabinets.