I suppose the subpage is read (from the TS DB) every time the editcount is called
As far as I know, there is no access to the page texts in database, you have to get it via http (or WikiProxy/River's thingy, which still is slow). So it wouldn't work as expected.
I propose to do this in Interiot's way, by commenting edits in talkpage (or, maybe, userpage too, since some users may archive their talkpages pages by moving them). You type something like '[[:tswiki:smth|Editcounter opt-in]]: graph1:yes, graph2:no, ...etc', and you may repeat it (change options) at any time. Then you can get the latest opt-in comment by one SQL query.
— Kalan