The documentation tools so are seem to be focused on getting data out of extended properties, not necessary making the population of those values practical.
At this point I feel I have to mention the Red-Gate SQL Doc is a tool that does make maintenance of the extended properties easier and no, I don't work for Red-Gate.
Since I wrote this article it has been hammered home to me just how important this stuff is. Let us suppose that you are a standard e-commerce site such as Amazon. Yes, your revenue comes from selling items but when you consider data you have so many other revenue streams available to you:-
1. Driving cross-sell/upsell and next logical products
2. Selling data to your suppliers
3. Selling data to government organisations. For example, in the UK we have the Office for National Statistics. A website with a broad product and customer footprint can inform governments of buying trends and spending habits (consumer confidence being in the news a lot a present)
I know of some catalogue retailers who liaise with the police to help spot/track fraudsters.
All this is only possible if you have a catalogue of your data, including its lineage. You are on a hiding to nothing if the information is locked up in peoples heads.