The topic of using a Camelbak (misspelled as a Camelback) pack as an enema bag has been discussed elsewhere on the Zity.
There are a number of sizes for the bladder from 1.5 liters (50 fl oz.) to 3.0 liters (100 fl. oz). I own a 2.1 liter (70 fl oz) Camelbak. So size choice is the first item. And then there is the matter of properly filling it and burping it to get rid of the air in the bladder.
Second, you have to remove the bite valve which provides a seal on the tube. Normally, you'd bite and suck and the differential pressure will draw the liquid into your mouth. Instead of the bite valve, you have to replace it with some sort of nozzle. Of course, when you do this, you need to install a clamp because if the hose level is lower than the top of the water level in the bag, it will flow out.
You can be as creative with the nozzle as you want depending upon how easily you wish to retain the enema before releasing. Some sort of inflatable nozzle that you could later release would seem preferable.
You might want to install a check valve in the line so that the flow is one direction only. Or more directly, a Higginson pump with which to pump the enema in. Higginson pumps have two check valves so the direction is always one way (in). The pump takes of another problem: as the water flows in from the pack on your back, the water level drops in the bag and begins to rise in your rectum and colon. Depending upon the pack, the water level might reach an equilibrium level (the top of the water level in your colon equals the level in the pack).
Finally, the pressure in the colon could force the enema back into the bag (the reason for the check valves). Although most check valves work well, I wouldn't want to depend upon them to keep the enema from flowing back into the bladder and then later using the bag for drinking purposes. I would have a dedicated pack bladder for the purpose of a hiking enema.
As others have pointed out, there's a lot of liquid and you might exceed the volume holding capacity. I'm not a diaper wearer but I do like to occasionally hike in the woods with the intent of taking an enema in the wild.
Be safe out there.