The OGC's opinion letter is here.

Footnote #2 of the court's decision is important. The insurer apparently did not timely assert one of the exclusions that is found in the prescribed UM/SUM endorsement that would otherwise negate coverage for employees or volunteers of a named insured injured while using their own motor vehicles if those vehicles are not specifically insured for SUM coverage under their employers' or principals' business auto policy:

This SUM coverage does not apply:

2. to bodily injury to an insured incurred while occupying a motor vehicle owned by that insured, if such motor vehicle is not insured for SUM coverage by the policy under which a claim is made, or is not a newly acquired or replacement motor vehicle covered under the terms of this policy.

The "insured" as used in that exclusion would be the church employee in the OGC opinion letter and the volunteer firefighter in this case.

Oddly, on its face, this exclusion seemingly would not apply if the employee or volunteer of the named insured were using someone else's vehicle, i.e., a non-owned vehicle. I guess the underwriting intent is to not provide SUM coverage for the use of owned vehicles that employees or volunteers regularly use. Nonetheless, the implied requirement that the employee or volunteer be acting within the scope of his or her "employment" by the named insured at the time of the accident remains, regardless of the vehicle's ownership.

So, an employees who is injured in the course of his employment while driving someone else's car may be entitled to SUM coverage under his employer's business auto policy if it includes SUM coverage? Under this decision, it appears so.