Yes, there's a fairly simple proof, specific to the B3/S23 Life rule, not true for all similar CA rules. Unfortunately this comment box is too small to contain it... and the proof only applies to structures traveling through empty space. If you're allowed to have pre-existing live cells ahead of the glider, there are known "wires" that transmit information diagonally at 2c/3, easily fast enough to get ahead of a glider and stop it.
