I’ve written a separate plugin for this, which handles hex and octal as well as decimal numbers.
I’ve had a script in my config for a while to perform arithmetic of the selections’ content, I often use it in combination with the # register (
<c-r># to paste each selection’s index in insert mode, very useful).
One nice thing about it is that it only uses one subprocess so it still works well if you have thousands of selections. It’s not very flexible though