Silly question really, as I'm 99% sure you can, but wanted to check if anyone had done it before?

I am flying DL premium select west (upgraded from MC) & then on the same ticket VS upper class east (upgraded from Premium), but for at least the latter flight I want to use my SAS ff no. for it to count towards the SAS Millionaire promo (&, sadly, I believe even the meagre earnings thru SAS for a DL-marketed PS fare class flight will earn more than it would as a DL Diamond).

For the former, it'll make sense to leave the Skymiles no. in there for the chance of a complimentary upgrade to D1 & just pick up a short DL segment in between the two for SAS.

Anyone done this? Multiple Diamond line agents acted like I was on drugs when I asked if this was possible/the case.

You can earn. However it will likely take manual intervention. It will post to SAS as an award fare class due to the upgrade. They'll then have to go and manually repost as originally paid fare class.

Thanks for the insight! Do you have any idea what that would require? Presumably I would just have to reach out to SAS in that instance & they would confirm with DL behind the scenes? Not sure if submitting all the tickets/evidence of fare classes directly to SAS would help expedite it?