You should note that many large Relational DBs (Oracle and SQLServer) have XML datatypes to store data within a database and use XPath within the SQL statement to gain access to that data.
Also, there are native XML databases which work very much like SQLite in the sense they are one binary file holding a collection of documents (which could roughly be a table) then you can either XPath/XQuery on a single document or the whole collection. So with an XML database you can do things like store the days data as a separate XML document in the collection... so you just need to use that one document when your dealing with the data for today. But write an XQuery to figure out historical data on the collection of documents for that person. Slick.
I've used Berkeley XMLDB (now backed by Oracle). There are others if you search google for "Native XML Database". I've not seen a performance problem with storing/retrieving data in this manner.
XQuery is a different beast (but well worth learning), however you may be able to just use the XPaths you currently use with slight modifications.
