Recently I’ve created mods for “Besiege” (editing Assembly-UnityScript.dll with JustDecompile with the creators’ consent), but I’ve found the need to edit UnityEngine.dll.
Support is full of it. This is a community site where volunteers with no official connection to Unity write answers to questions about the editor. There’s nobody here that’s got the knowledge or the authority to answer questions about what you’re allowed to do with Unity’s dlls.
From general experience, though, I assume that Unity having a copyright on their code would mean that redistributing it, even in edited form, is really, really illegal and could land you in jail. So, yeah. Probably don’t do that unless you get a “yes” from an official.
Contact support again, and make sure to explain that it’s UNITY’s dll’s you want to edit, not Besiege’s. In fact, don’t mention Besiege at all. In the mean time, see if you can glance anything from here.
So the community here are now being given the authority to answer legal issues concerning Unity’s core IP? That’s nuts! I can only imagine they have completely misunderstood your question.
FWIW, the licence agreement states “you agree not to disassemble, decompile or reverse engineer the Software, in whole or in part, or permit or authorize a third party to do so, except to the extent such activities are expressly permitted by law notwithstanding this prohibition.” Unity Software License Agreement 4.x
It would be hard to see how modifying UnityEngine.dll wouldn’t put you in breach of this.
This might come under the requirement of the Source-Code license but I think this is reserved for big industry players.