I did something like this in LedgerSMB. While we use EAV modelling for a few things (where the flexibility is needed and the sort of querying we are doing is straight-forward, for example menu nodes use this in part), in general, you want to stay away from this as much as possible.
A better approach is to do all of what you are doing except for the data columns. Then you can (shock of shocks) just create the tables. This gives you a catalog of what you have added so your app knows this (and you can diff from the system catalogs if you ever have to check!) but at the same time you get actual relational modelling.
What we did in LedgerSMB was to have stored procedures that would accept a table name exists ('extends_' || name supplied). If so would add a column with the datatype required and write this to the application catalogs. This gives us relational modelling of extended attributes. At load time, the application loads the application catalogs and writes queries as appropriate at appropriate points to load/save the data. It works pretty well, actually.
