Varro mentions the possibility:
De Lingua Latina 9.75.4ff.
obliquos non habere ut in hoc Diespiter Diespitri Diespitrem, Maspiter Maspitri Maspitrem. ad haec respondeo et priora habere nominandi et posteriora patrici esset casus. ut ovis, et avis. sic in obliquis casibus cur negent esse Diespitri Diespitrem non video, nisi quod minus est tritum in consuetudine quam Diespiter; quod [in]nihili argumentum est: nam tam casus qui non tritus est quam qui est.
However, outside of grammarians, there is no literary record of the oblique forms, nor could I find them in inscriptions. They exist only in nominative and vocative.
As TKR noted below, Varro is saying that they should exist, and that others (the anomalists) say they do not exist. This section comes from book 9 of De Lingua Latina, though, in which Varro is presenting arguments against anomaly; this makes it a theoretical engagement, not words that ordinary Latin speakers would have known and used.