I got my hands on a UP-D898MD on Thursday, and after some head-scratching bugfixing, I'm pleased to say it now works reasonably well under Linux, incluidng custom print sizes and status reporting.
It's worth noting that other the other members of the '898 family (the non-MD versions, as well as the UP-X898 and UP-D898DC) might have their own quirks, but only time will tell.
There are still some missing features I haven't yet figured out -- For example, how to control the copy count and things like brightness/contrast from the host side.
Along the way, I landed bugfixes that affect other "Sony UPD Neo" printers (such as the UP-DR80MD and UP-CR20L).
To take advantage of this support, you'll need to grab Gutenprint out of git (or a snapshot/release dated after 2020-11-07).
Oh, the aptly-chosen test print is from Episode 91 of Sanesparza's "How To: Comics"
Update 2020-11-08: Fixed copy count