Sure. I was, separately, just saying that there are ways of having a DLL which is updated while MT4 is running. What you raised, as I understand it, is a different topic to the original one. It is (a) possible to do something equivalent to #import "http://myserver/mydll.dll", and (b) possible to update a DLL while MT4 is running - provided that there's a layer between MT4 and the DLL which actually contains the functionality.
You could combine (a) and (b) - download two DLLs, one of which is a shell, and the other of which is then further updateable while MT4 is running.
I'm curious as to how this can have been possible.
What's changed is that MT4 used to create a new thread to handle each tick, and now instead assigns an EA to a single thread throughout its lifetime. But, in order for you to be able to delete the DLL, MT4 must, in each individual tick/thread, have been loading the DLL and then unloading it at the end of the tick/thread. I've got a vague memory of that behaviour, but the performance of DLL calls will have been significantly worse than it is now.
I might dig out a pre-509 build just to satisfy my curiosity.