There are probably little if any risks with this as long as the button is pressed slowly and the whipped cream does not blast out of the can too rapidly.
There is no danger of the propellant being absorbed by the colon.
The propellant in Reddi-Whip is nitrous oxide. You won't absorb enough to get high from it, as that requires a relatively large amount that must be absorbed by the lungs to get into the bloodstream.
You can't get an embolism from the gas from the colon unless you actually rupture the colon wall and the gas goes into a large vein. Capillary bleeding will not pose a problem.
You CAN get an embolism (actually the air will blow into the peritoneal cavity, NOT a blood vessel) with high pressure air blown into the vagina because the air actually blows through the uterus and into the peritoneal cavity.
I wouldn't consider this to be extreme, but it is fun! ;D ;D ;D
Just be careful and don't empty the can too fast. As long as the nozzle is just inside the anus, any extreme pressure should just get released by blowing out the anus.