I don’t think a BPF after the LNB would be productive, as the LO drift would make the signal wander around relative to the BPF. If you get an LNB with a reference-clock input ($$$), you could do it, but even the PLL LNBs are gonna have some drift.
It’s my understanding that the SDR should have no trouble as long as the signal strengths across the passband aren’t totally blowing each other out. You’d need a filter if you wanted to knock out some signal that was swamping its neighbor, but as long as it all fits within the ADC’s dynamic range, the software demodulator should have no trouble with it.
Now, the dedicated receiver chip on the DC3, we’re assuming is a DVB-S2 chip, which should also be quite capable of handling whatever mess you normally find on satellite bands, but what techniques it uses, we have no idea of. I don’t believe there’s any need for anything you wouldn’t normally find in a satellite receiver setup.