Back plates and wings can certainly be used for training a new diver, a BP&W is after all just a type of modular BC.
There are enlightened BOW instructors here in SoCal (and other places too) that routinely teach BOW students in a BP&W.
BP&W are however relatively uncommon, and that can lead to practical difficulties.
Proper initial setup of a BP&W is a bit more involved than a generic jacket. BOW instructors are already under paid and over tasked, so any "extra" work can be unwelcome.
Many instructors are not well versed in BP&W's and therefore aren't well equipped to help a BOW student with a BP&W.
There is a very common misconceptions that BP&W's are for "Tech" only, that BP&W's are "complicated", that BP&W's are unnecessary or unfit for Recreational Diving.
I'd suggest you speak with your instructor. He / she is ultimately going to have to be comfortable with what you use during training. You can of course look for an instructor who is familiar with BP&W's
Let me know if you like a recommendation on a plate, harness and wing appropriate for your needs.
Tobin