DL3: SDI in AND Camera Out (via SDI)

I am working on a project and here is my scenario:

I have an SDI feed going into each DL3, but I would like an option to select whether or not that feed passes through to the SDI out.

In my scenario I always want the DL3s to output their camera (via SDI) while having an SDI feed coming in.

From an engineering/hardware/software standpoint is this possible?

-BC