Yes, I put my starflash mirror behind the plastic insert.  I left it in its plastic bag to mimimize scratches.  I had stuffed a bit of paracord back there also, but removed it in favor of the water bag.  I bet a JetScream would fit back there but I do not have one.  I do have a flat ACR whiste in the tin, but I know it would fit along with the mirror and water bag.
Also my $49.99 sheath came with the 7" Becker...made the sheath a much better bargain.  <img src="/images/graemlins/wink.gif" alt="" />