Did you try it? In 2 minutes you should have had your answer...

A DDR doesn't go through the RPD and therefore its columns do not match any LSQL formula referencing RPD objects. DDR shouldn't be considered as normal objects providing standard features, this is just one of the many things you can't do with a DDR.

I didn't test in 12c because I don't have one available, the test was in OAS 6.4.

Nothing has ever been added to DDR, it's almost surprising they never dropped DDR fully. It does the bare minimum and has minimal support to variable, that's about all you can do in DDR.

It's generally easier to consider that DDR doesn't exist