Good idea..
Don't use memo, use the market order book.
For example, if you want to sell/buy something, you create a webpage or blog post or google doc or whatever static with detailed description, sign with your key, and encode the url of the page into a number, then you submit a bid order in a special market with the number as price and/or amount. Others see the order, decode to the url, saw your request, then create a proposal for you.
Another way, (ab)use the committee member info data, can set a url there, update fee is 2 bts.
Yet another way, (ab)use the ASSET info data, can update the description, fee is 20 bts plus 2bts/kbyte of data. Oh, you need 5000 BTS to create an ASSET if not already did.
The golden grail is the "custom" operation, possible to do anything with it.