For discussing it spec-wise, commenting on this issue is fine; Alternatively subscribe to and start a discussion on the BV mailing list.

For prototyping it in the BV reference implementation Hibernate Validator, I suggest you open up a separate issue in the HV project, state you'd like to work on, get started hacking on it and submit a PR on GitHub as soon as you have something which shows the general direction so you can get feedback early on.

Many thanks for getting involved!