[ol][li]Resize image based on max(width,height)
I don't think you need to use any complex scripting for this. If you tell Opus's Image-Convert command that you want the width & height to be 2048 but also want the aspect ratio preserved, then it will resize the images to fit into a 2048x2048 box, keeping the aspect ratio. I think that's what you want?
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[li]Rotate based on a filename pattern
That, on the other hand, does requires scripting. Opus can do the rotations but you'll need some kind of script to work out which files you want to rotate and tell Opus to rotate only them.
You could use my "Abusing" Rename Scripts technique (as mentioned in your post) to make the script live inside an Opus button, but where the script lives is an afterthought compared to writing the script itself. It's usually easier to write a normal script (e.g. using VBScript and a .vbs file) to do what you want, and then when it's finished convert it into an Opus button if you want that as well.
I've never looked at GIMP's batch/script support so I don't know whether or not it would be a better choice. (I've done some Photoshop scripting and it could do it, FWIW.) Assuming scripting is possible, I'd say it makes sense to make the script work with whichever tool you are likely to be using when you want to perform the task. If I wanted to launch it from Opus, while managing files, I'd probably write some VBScript which took the filenames and selectively ran the Opus Image-Convert function on them. If I wanted to launch it from a tool like GIMP or Photoshop, while editing files, I'd look into scripting them.[/li][/ol]