I could not find the option to include the DEM and RPC files in Orfeo (Geometry tool). Please, review the attachment.
Can you open an issue to the otb issue tracker please?
I think that you'll be a better chance to have an answer.
I suspect that it is due to the way that the otb applications are integrated in QGIS which impose sompe simplifications of the applications configurations. Someone on the issue tracker would be able to confirm this.
Note that there are ongoing developments to improve the support of otb in future versions of QGIS. You can find more information about it here: