Easiest way to create / maintain many<->many mapping tables in EBS?

We are building an R12.1.3 Projects extension that requires a many<->many mapping table between Transaction Source and Expenditure Type

e.g.

TS1, ET1

TS1, ET2

TS2, ET3

TS3, ET3

i.e. it defines the valid expenditure types for a given Transaction Source.

I can't think of any existing e-business tech that lets you enter many<->many mappings where each side of the mapping is validated against a particular value set.

Our current two best options are to either create our own key flexfield to enable us to enter the valid combinations - but then we need to create a custom table and custom maintenance form, or else to create a webadi integrator against a custom mapping table - and to download/modify/upload the mapping table using Excel. We'd like a solution that allows maintenance in the Forms / OAF pages though.