Maybe with a script ? I don't know much about scripting , but maybe if you controll a settlement that has a hidden resource , you could try to add 1000 denarii every turn (or something like that
)
Or a (new/custom-made) building that gives a bonus , but needs a hidden resource to build 
EDIT : I think it should look something like this :
Don't try this , cause there's a fault in this most likely ; it's just an example ..
You can see that this building requires the romans_brutii , and you need the hidden resource rome .. So this building should be only build-able if you're the brutii , and you control a region that has the 'rome' hidden resource .. It gives a 'taxable_income_bonus bonus +5' ; so a +5 bonus to tax income (normally)