Calculating and granting Idle rewards

I'm working on an idle/tycoon game and trying to grant currency based on how long the player was away, up to some limit, and based on previous gameplay.

This seems like something the reward system should be designed to do...grant something based on time period and another variable. It seems to be way more rigid than it should be. It also seems to have no functionality for randomness in the rewards, like a chest that gives between 1000-2000 coins for example. Or a 1% chance of a certain inventory item. Classic game mechanics that I'd expect in this kind of tool.

I'm thinking of doing the 'idle production' thing by storing the last playtime in a Mutable property of an Inventory Item, and having in-game code grant currency when the game is reloaded. This feels like a hack though, are there any better ideas out there? Shouldn't we have mutable properties on Game Parameters at least?

Thanks!