I don't think one can really cover all those different cases of manually adding controls and I also don't think that's the right way. Too much complexity, too much ways to really mess up addons.
My oppinion is: Give the authors some time edit their menues and help them doing that if possible by converting their menues for them.
For that I also initially thought about making an addon to convert the settings, but after looking at some really special menues I decided I would never be able to handle all the different ways of doing special things with the controls (at least not with my only moderate lua skills).
The solution I came up with is a non-lua based code converter, which can currently cover all cases where only the LAM-1.0 API was used, ignoring everything else. With that I converted 4 addons of other developers, doing the special things manually after using my converter, which wasn't much work. If anyone wants to you can use it yourself:
Converter
It's still under development, I have some ideas to make it better and more comfortable (converting headers to submenues for example).