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I thought about writing such a plugin before, but not using Scintilla directly, I think I even might've wrote a little code for it. I was looking at using GtkHex[1] and messing with the main documents tab like the Devhelp plugin does. Getting a hex viewing widget into Geany is pretty easy, but hooking it in to handle different filetypes and whatnot would probably be a little harder.
Not sure what you mean by handle filetypes, I thought the point of something like this would be to handle it as hex, no matter what the file was?
Cheers Lex