Hi Craig,
I think it's very sweet that you are proposing with your mother's bridal jewelry. I think your woman will be very touched by the sentiment.
As far as your question is concerned, I would just propose with the engagement ring band. You can explain during or after the proposal that you are looking forward to placing the matching band on her finger on your wedding day. (To get a second opinion, I asked my husband what he thought, and he agreed with me).
