I was wondering if it is detectable to modify the d2gfx.dll file to allow multiple versions of Diablo2 open without the multi copy warning?

Rather than editing it yourself, you can download ones that are already edited from multiple sources - I think rgx mod includes an edited one. Yes, blizzard has the ability to detect these things, but they seem not to be banning for them atm, or really, never have banned for them.

I know some come pre-edited. I am no script kiddie and if I wanted to I could easily edit it myself.

Just for backup: Are there people that use this right now that have not been banned for it? Or anyone that has used it and was banned for it?

I use an edited one and have not been banned from it :)

no one was banned yet

mabye blizz allows multi d2s