Or just create ad hoc tables with user fields. Quite often it's not that a customer has n different fields for n entities, but a few that apply to the majority

Or just create ad hoc tables with user fields. Quite often it's not that a customer has n different fields for n entities, but a few that apply to the majority (like internal ERP ids, classifcation etc.). Put them in a few tables, index them, join them. If you don't want to parse internal DB descriptors, create a set of "schema" tables to build queries from.