Could my orthodontic retainer work as a bite guard to protect against nighttime teeth grinding?

Yes and no: Some types of orthodontic retainers will protect against nighttime grinding, some will not. Ask your dentist or orthodontist for the type that will.

Only if : Only if retainer goes across the chewing surfaces of the teeth and is adjusted to eliminate interferences. On the other hand, an occlusal orthotic makes an excellent retainer. Sports mouth guards should be used for that purpose only.

Temporary solution: The nite guards are made from a much more durable acrylic material than retainers. If you are broxing and grinding on orthodontic retainer, you'll damage the retainer. Ask your orthodontist for nite guard. Night guards should only be worn at night, during the day you can use your regular retainer.

Maybe: Depends upon type of material of the mouthpiece, upper or lower, thickness, hard or soft, cover all or just some of the teeth. You may get several opinions on this topic.