You mean transaction v6, not NU6. And yes I agree on both points: that it would be useful, and that it is late in the NU7 schedule to add it now. It could reasonably go into NU8, along with the mechanism(s) for lockbox disbursement that @joshs and I intend to write a ZIP for.
(The NU numbering here assumes no NU inserted before what is currently NU7.)
Incidentally — although the main reason we didn’t add this is that we didn’t think of it — I think there would have been an argument not to add a mechanism for voluntary donation to the lockbox without having nailed down any disbursement mechanism, on the basis that potential donors would have insufficient information to make an informed decision to donate. (It is somewhat analogous to ZIP 233 without 234.)