City Center Building with Location Requirement?

Folks, I created a UB which is a replacement of Watermill in City Center for my new Civ. Since my new Civ design is a strong naval one, I want to boost yields from that building to fishing boats but restrict the building requirement to be City is coastal.
First hunch was to find out what the Australia trait does. But that was a plot yield change modifier. While mine is a building prereq.
Then thought what about coastal wonders. Ok they only required AdjacentDistrict being "Harbor" or RequiredTerrain being "TERRAIN_COAST". That only makes sense when the building needs placement. Same as the "coast" boolean value from Beach Resort I found the closest to what I want. But didn't work.
Reading through all the buildings, and the sql Schema, the only prereq terrain/feature for a building that doesn't need a placement is the freakin "RequireAdjacentRiver" for Watermill. No resource prereq as projects and units, no terrain requirement as wonders that need placement...

Impossible? Please help smart people, there must be a way.
PS: either restricted to coastal cities, or coastal cities/existing harbor test any, or city has sea resources are acceptable solutions.

 

I figured... Also seems resource required building is impossible without LUA.

 

I'm in the middle of major structure overhaul of the linked system. Hope to have it published this weekend. Re-work of the system will vastly simplify using the system for other mod-makers and will eliminate the oddball corner-case mod-conflict issues that arise because mod and mod-file loading orders are not as stable in Civ6 as we would all like.

 

I've updated the LeeS' Dummy Buildings Utilities Gedemon linked to upthread.

 

Just had a chance to look at your dummy buildings utilities. They are absolutely awesome. But one question I have: in civilopedia would the real building show a prereq building as this dummy building?

 

So long as no other mod interferes with the UI files of the "Systems" mod, no.

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Every once in a while a city production qeue will show a 'real' building as being greyed-out (unavailable) for a city, but as soon as UI systems refresh even this will go away and the production panel will not show the 'real' building as a possible (though currently unavailable option) for that city. This has to do with Firaxis and the lag between UI and actual gameplay states for which there's nothing I know of I can do.