I think the Fluke 287/289 both use an STM32F of some kind.
How hackable are they?
Firstly, someone has already mentioned that the firmware is likely to be protected, it is near impossible to read out.
Secondly, there are over 100 variants of the STM32F line of micros. Even if you know the specific micro, with all the peripheral options, knowing what all the io pins are doing means reverse engineering the entire circuit.
Thirdly, The original source code is likely to be in high level language like C. So the amount of binary/assembly code is likley in 100s Kbytes. Then you have compiler optimisation to get your head around.
Simple answer - not very. Only the really, really, really, really determined will even try!