I haven't tried it out yet, but it looks amazing.
An idea would be to have it read through your AddOns folder to see what addons are installed (just reading foldernames should be enough) and then read from WoWInterface and possibly Curse to see if one has been updated with a link to the addon's wowinterface/curse page.
This is different from addon clients in that it doesn't offer a straight download via your program, but instead links to the webpage itself and therefore does not leech bandwidth.
You wouldn't have to have a full list of addons displayed anywhere in the client, or do any checks apart from folder names. Version checks shouldn't be needed as you know if you've updated or not. Just having a box that says "these have been updated in the last 5 days!" should be enough.
Just an idea, it'd be very cool.