While looking at some of the price history data from EMD, I noticed that most tech 2 items move less than 100 Units in any given high-sec region in a single day. It should be possible too, to make a spreadsheet that uses a corporation's market orders to show how many of a specific item were sold on a specific day in a specific region.
It's a simple matter after that to show your market share by comparing how many of the region's movement was your items. For example, on 12/8/2013 81 Invul IIs sold in The Citadel, 12 of which were from LDIS giving them a 15% market share. You could zoom out the time frame to month, quarterly or higher too if you keep track.