I have 20 level 90+ toons all slowly making their way to level 100. Some slower than others, but I've gotten them all at least their barracks setup so they also generate Garrison Resources in the cache and the cache unfortunately caps out at 500 resources. What I've looked for is an addon that at the very least will display the amount of Garrison Resources in the cache so I can go and collect them and keep it from capping and wasting resources.
The resource cache itself can't be queried for other toons; I don't even think you can query it for the toon you're logged into, only open it.
But capping is not hard to avoid. Garrisons generate 1 resource every 10 minutes, so that's about 3 and a half days of not clicking the cache before it fills up. Presumably you're logging on at least twice a week in order to juggle missions and whatnot.
If you want an actual resource count, you'd have to just estimate it. A "data broker"-type plugin could take note of when a given toon clicks the cache, and store the current time. After that, the toon's approximate resources can be queried by taking the difference between the current time versus the stored last-clicked time, and doing some basic math. Might even be able to build that into Broker_Currency or something similar.
...except that Broker_Currency keeps track of what you have, not what you might have if you perform a certain action in game.
That's a good point; I suspect that no matter how many disclaimers and caveats were displayed, B_C would still be getting tickets about how the Predicted Approximate Garrison Cache, Maybe column was "buggy".