TomW:
Let us know what you decide on for a blade. Clearly, from my earlier post, I'm hoping you'll choose a folding boat knife.
There are aftermarket nylon sheaths that attach vertically or horizontally to a belt or strap. Not sure how your lifejackets are designed, but you may be able to attached a nylon sheath horizontally to a waist belt. You could further secure it to the waist belt using a brass rivet. If you do sew a pocket to the jacket for a knife, consider sewing it on the inside of the jacket (against the wearer's chest). This helps keep the surface of the jackets snag free. I have had more close calls with boat crewmembers entangling lifejacket buckles, tabs, clips, etc., in running lines and nets.
As you know, surviving boating emergencies usually means staying alive long enough for a rescue. This means that you'll need to stay with the boat, if afloat, and stabilize this floating platform until assistance arrives. If your sailboat rolls, turtles, or snaps a mast, you'll need to clear rigging to stabilize the floating platform. A pointy Spyderco folder or Mora fixed blade won't do squat to open a shroud/stay shackle, remove a clevis pin, or clear a knot out of wet dacron line (rope). This is why I suggest the boat knife or Bosun's knife, which is made to perform all these tasks.
Lastly, you don't have to spend a fortune to outfit the jackets with boat knives. As far as lasting power, I purchased my Currey Ltd. Bosun's knife (below) in 1980 for $18 and it has seen rigorous service on three oceans.
Cheers,
TT2Toes