explicit forward-to-debian tags?
This may not belong to sponsoring but to LP or other UIs that pop up in the packaging workflow, but some clear indication that a patch is considered better fit for going to Ubuntu via Debian would be helpful.
There are quite a few sponsoring requests that linger because they are too tiny and insignificant (in the whole Ubuntu ecosystem, not to the contributor, for whom it obviously is important) to act on to worth the hassle and the gratuitous delta.
If there were a Tag (although a bug-state or branch-state would be more official sounding) that meant this is thought to be better sent to Debian first, it could be easily searched for in BTS sending sprees (if one is so inclined) and be filtered out from the sponsoring list of substantial patches.
This is less of an UI issue and more process-related, and one with no clear answer - one sends away contributors to make a more significant effort and delay their gratification in the same time by possibly months, but it is being done already I guess, just not very formalized?
A tag is a stronger indicator of action to be taken than a comment in English which is usually not worded very assertively to not discourage the contributor.