Well it really depends on what you're trying to do, some projects are fairly accessible while others would require more digging. Some of the do-able projects I've seen include changing out the USB connector for a different one, powering the calculator from the wall instead of the battery and adding wireless charging.
However, I think when people talk about "modding their CE", they usually mean adding more ram or flash. I'm afraid for those kinds of mods, you're pretty much out of luck. In theory, the eZ80 could map up to 16MB of memory but you couldn't physically hook up the additional address lines required (since they aren't exposed outside of the asic) and even if you could, the OS wouldn't support it.
The CE really isn't as interesting as older models in terms of modding because the entire thing is just 2 chips, 1 of which is the asic containing the ram and cpu (can't feasibly upgrade that), the other is the flash chip which can't be swapped out for something bigger.
One cool project I've seen is to
add a Trinket M0 flashed with the TI-Python firmware to add Python to a non-python CE. Those little boards could easily be squeezed into the case.
Another fairly easy but boring project could be to populate the exam mode LED circuit to essentially turn your 84PCE into an 83PCE.
