There have been some cases but in most it will be a rescuer that does the cutting and not the victim.
Inside or outside is your own choice. Both have pro's and con's.
In South africa we used to do an excersise with trainee trip leaders where they had to hang onto tubular webbing with a tape knot in the end. The trainee was then lowerd into some rather violent current and had to hold on with one hand and with the other get their knife out and cut themselves loose. To pass the excersize the trainee had to bring the knot to the instructor (me). This excersize is the only way you will understand what it really feels like to try and cut yourself free.
This is a good excersise but please do not try it by yourself and also do not try it with out getting a proper protective glove for the arm that wil be holding the webbing/tape. a heavy duty welding glove works well but it must cover your arm all the way up to or past your elbow so that you do not cut yourself to shreds and bleed to death. Best option is to do a swiftwater rescue course and then on the course talk with the instructor about knives and the what,where,how etc
If you do decide to paddle with your knife on the outside of your pfd do not tie it onto yourself with long strings, if you do this and drop it in current it will act the same as the blade on a spinning fishing lure and can shred you easily.
River knives should also only be used for rescue situations, using it for lunch and things like that wil over time blunt the edge on your knife and then when your mate needs you to cut him free your knife might be to blunt to cut through the rope or tape.
you can also have a look at:
asafrica.ning.com/profiles/blogs/paddling-gear-info
cheers