I plan to make an web application using XPages for the design (plus the Lotus Notes Elements), but instead of storing the data in Lotus Notes, I will store the data in a RDB
The short answer appears to be "yes but no" .
XPages is JDBC compliant, so you can connect to anything, including Oracle. The snag is, you'll probably have to write the code yourself. Apparently there are plans to open source a JDBC-based Relational Database XPages DataSource but there's been no activity as such. Find out more