altering improvement upkeep
On freeciv-3.2 there's Output_Bonus_Absolute (#43593) which you could use to adjust output of the central tile. That approach might reduce client side help / popups quality (those adjustments don't show up as 'building upkeep')
Oh, so I could just "add" -1 gold that way? Interesting. Not reflecting the change in the building info directly wouldn't be that much of a problem, since other effects can't do that either (like Adam Smith or Colosseum). I'd add it to the description instead. Though I would like it if those parts of the display could be made dynamic in the future, for exactly those cases and significantly more convenience and clarity.
Is there any way to change the upkeep for specific buildings by a specific amount? The only supported effect I found is the Adam Smith one and it's very specific in it's effect. I have two ideas I'd need that for. Hospitals are an improvement I added to grow cities above 24, with the tech Public Healthcare I would like them to make one citizen content but cost one more upkeep. I'd really like to have that in the game, because I created the tech specifically for it. The second thought was an alternative effect for the Great Library, to make Libraries and Schools cost no upkeep. I might be able to do that one if I can combine Upkeep_Free effect with a custom tag for buildings, like it works for units, but I didn't check yet if that is possible, let alone if it would work in the intended way.