At one point, I asked UG directly if there were any plans to allow the creation of .mdl files dynamically, and they said no. It wouldn't hurt to send them a request for this feature -- perhaps if they get enough requests, they will at least consider it.
If you make your modifications to the blank models in PostRunFn, you will at that point have the complete list of loaded vehicles, both mod and stock. You could at this point also generate more blank model files if you didn't have enough. These new files wouldn't be loaded into the game unfortunately, but they would be available on the next game load. With the new API's, you could presumably pop up a window advising the user to save the game and reload if you ran out of blank models.
You could have a look at this mod:
https://steamcommunity.com/sha…iledetails/?id=1970151023
It increases production over time, although the increases are based on use, rather than the actual passage of time. As the author points out, this mod uses a custom version of industryutil.lua, which is less than ideal, but it can be made to work.
You can generate your own cash transactions. See my Taxes & Subsidies mod (actual title is 'Revenue Adjuster'). The only downside is that they show up on a separate line on the financial statement. If you're willing to live with that, then you can adjust the income on a given delivery or maintenance transaction to be whatever you wish by generating a new transaction for the difference between the game generated amount and the amount you want to target.