Does the bridge length or tightness have any affect on squirt (cue-ball deflection)?
No, unless the bridge length is really short and the bridge fingers are very bony and have an extremely tight (i.e., non human) grip around the cue. This reasons for this are similar to the reasons why the grip can have no practical effect during tip contact.
Even if the bridge were perfectly rigid, it would still have absolutely no effect for bridge lengths beyond about 6-8 inches. The following video (at the 2:32 point point in part 2) shows and explains why visually: NV B.96 – Grip and bridge technique and advice.
And Diagram 4 in the following article gives some additional experimental proof related to endmass:
“Squirt – Part VII: cue test machine results” (BD, February, 2008)
Only mass and support within the 6-8″ closest to the tip can make a difference, with the mass or support farther from the tip (within the 6-8″) having less effect.