I understand this as a guaranty, that it's not used for spamming. (See also update (3) )

And I don't expect many would object about an extra mail every x years.

Additionally I seem to remember - maybe incorrectly - that these emails were used for sending warnings after PM was hacked.(?)

My suggestion

Just amend the policy with a phrase° that the email can be used for exceptional personal emergency contacts by the admins, but never for any kind of advertising.

Reasoning:

• no code required
• any technical solution will need an explanation anyway
• any technical solution will only effect new-commers
• it's a social, not a technical problem
• techs tend to try to solve social problems with technical approaches
• emergency contacts are affected by the same rot like normal contacts (5)

Concerning monks who haven't seen the policy change, because they were away in the meantime:

• "It's easier to ask forgiveness than it is to get permission"

just do it till someone objects, and if this really happens, add him/her to a manual exception list (which will be very short)

Updates

°) I leave the wording to native speakers and victims experts of US I-will-sue-you culture*. ;)

*) LOL , that's an epic boomerang 😂

3) from Create A New User

Please note:

We are not going to send you junk email "member updates", sell your address to spammers, or display your email address for people to see. This email exists so that you can receive your password, and/or retrieve it if you forget. There may be future email functionality, which you will have the option to turn on from your user settings page.

4) your mother is doing fine ;)

5) but are rarely updated. I personally know a case of an emergency last will written in WW2, leaving everything to a long forgotten former wife not seen for 40 years.