He's trying to improve interface. Warning messages are better, 'cause they don't push up (or look anything like) chat messages.
Then again, if I imagine it correctly, it won't be all that complex by using some dummies. Depend on the restrictions that trigger your warnings.
Let's say we have a restriction that is: Players cannot build CoolBuilding within 1500 of another CoolBuilding. So, the detection is straightfoward, but then to call our custom warning, we must make something, that is unallowed by default, happen through dummies, such as ordering triggering player's dummy worker unit to build a farm on our dummy unbuildable tile, which will ofcourse trigger the inbuilt warning, which can be customized to: "Another CoolBuilding is too close".
All one needs is to find out which action triggers which customizable warning, and figure out how to make that happen through triggers, without any player noticing.
Actually, it's rather simple when you think about it. And it may even be worth the effort, if you're picky about details.