Update:
It appears that Msg++ 0.1 works on MSN 5.0, but not 0.9. Version 0.1 is available at
http://www.nmitchell.co.uk/programs/msg++/, and the source code is available too. This uses a different API to get in and do its work, which seems not to have been killed. You can port to this API, you will loose some features, but at least you will have an addon.