well they can be done but even if they are based on preexisting megabeasts they will require a bit of work as you will still be in for some "fun" even if you were to mod them to be friendly given the effects they can have and what they can be composed of.

Though may exprment with the idea myself later but not currently because I have my own modding shenanigans to worry about first

You can certainly make friendly mega beasts. you can even add both [GOOD] and [EVIL] tags for a bipolar Megabeast 🥲