Can they put a new building in there and not adversely affect everyone already past that chapter? As long as they can do that, I'm all for this. In the event they would only be willing to change buildings, a small list of what's already there in each chapter (starting at 11) with no mana output follows.
I'm just going by what I see on the wiki, in the tech tree and in the culture tab. But it seems to me that Torrin, God of Harvest could just have this feature added on. It's already a premium building and only gives culture. And since Woodelves starts with a non-premium mana option, maybe something could be done to modify Guarding Grimaces to offer seeds?
The only other premium not already doing something in that chapter is the Traditional Fishery, which would be a weird one to produce seeds, but at a stretch I guess for lore sake the Halflings could use seeds as bait.
(On the other hand, Pole of Donations could be reskinned and a premium version offered that does give seeds, as long as adding a building in to an already existing chapter doesn't cause problems with the code.)
In Elementals, the only Premium building not producing mana is the Butterfly Shrub. For the non-premium building, if they wanted to offer one, the Monument of a Peaceful Past? That way the Butterfly Shrub could offer twice as many seeds as the Monument, tempting people more with the Shrub and not making anyone who places the Monument in their city feel too much like they're being snubbed? I mean, premium buildings do generally offer better production/pop/culture with less of a footprint.
Amuni's only Premium non-mana producer is the Scarab Merchant Ship. Rather enormous all the way around... And the Decorative Palm is the only non-premium non-mana producer. ...I'm really starting to see what you mean. You have to rely on event buildings just to get seeds, it looks like.
Constructs: Premium - D118-e Research & D-034a Spy; non-premium options: D027-h Archive & Step Stones
Elvenar: Premium - Flow of Magic Statue; non-premium: both the Wolf Shrine and the Elvenar Harp Player offer only culture. There are only four cultural buildings in that chapter. Huh.
Embassies: Premium - Union Tower. The only non-premium building offered in the chapter produces mana.
Traders of Unur: Premium - Monkey Palace. Non-premium situation same as Embassies chapter.
At any rate, I do hope there could be some from that list they could just modify to offer the seeds.