So I would think you can train a dog to hold something And drop something.
It's like stay. They keep doing the stay until you release them. I would think they could be taught to keep holding the object until you command them to release it.
Honestly, I never tried to train a dog to tug. I just accept that she doesn't tug and figured in my situation that's good. I always found "drop it" to be more important in my life with dogs, whether it's a toy or my own shoe, or some dead thing they found in the woods, etc. if they can drop it...
Also your dog that won't tug, you mentioned is submissive, and I believe this is exactly it... my submissive dog won't tug much with me or the alpha dog, I think it may be another way of submitting by easily giving over the prize and not fighting about. The dog is trying to please by sharing...
Another random thought... some dogs respond or get really excited by noises... as in a regular ball might not be all that exciting, but a sqeaking tennis ball could be the best thing in the world. Toys that make noise are instantly recognizable and can also be used to grab and maintain a dog's...