I havent looked into it (there was a spell effect list somewhere, but I cant find it), but I thought that it might be possible, as they would sort of emulate the effects of riches from beneath and Illwinter.
However, my own attempts at spell modding have ended up working badly. My Pangaea "Carrion Festival" spell still doesn´t work (It spawns a lot of carrion which doesnt move due to lack of undead leadership. And manneads fail to show up
)
An alternative to the Riches from the Sea spell would be making it a summoning spell, and have it add an unmovable unit which increases productivity, but also death (and maybe unrest?) That´s a fairly straightfoward mod and doable without too much trouble.
In general, I think that the right path in doability is stick to special terrain units and regular summons. (unless someone knows how to make remotesummon spawn different units? or even add multiple effects to one spell?)