An orgasm from an enema alone is possible for me, however, it is an event that occurs for me during retention, not filling. In fact, my fiancé prefers for me to orgasm from an enema versus traditional male masturbation which receives disapproving remarks in our household, if not a spanking. If I cannot wait until she is ready to play with me and make me orgasm on her terms, an enema is an acceptable way to find relief even if it is able to produce the same result as masturbation for me.
I do not make regular use of a chastity cage, however, I almost always wear chastity during my enemas. My enemas are usually 2 liters of hot soapsuds with 1 heaping tablespoon of Dr. Bronner's soap or a bar of Ivory mixed into hot water until the water is very cloudy with visible soap bubbles and a slick slippery feel. I am supposed to retain my water for 10 minutes so after letting it run into me in the knee-chest position I turn over onto my back. I have just a few enema nozzles but my best quality one is my OHN delrin nozzle; I have the largest one. Either this delrin nozzle or an inflatable nozzle can induce an orgasm during retention, however, I do not currently have an inflatable nozzle.
At this point if I am not wearing my cage, the act of rubbing my legs together during retention just to clench against the cramps will stimulate an orgasm. Usually this is not very satisfying either because it has a *ruined* feeling, being produced mostly from light frontal stimulation with some clenching against the nozzle.
If I am wearing my cage, I will began to leak fluid profusely during retention with a larger nozzle, and began to have entire spasms throughout my lower body as I feel the cramping from the strong soap solution as it works it way deeper into me. It is almost like a pulsing feeling in my male part, where I will fill my entire 1 1/2 inches of cage and there will be a slight pressure but no discomfort. However, by this point the discomfort from the cramps is working its way very very deep into me and I clenching very heartily on the nozzle in order to wait for the timer to sound. If enough soap was used in the enema, and I have abstained from masturbation for a few days, I may have a wave of cramps that sets off what feels like a chain reaction in my lower body, causing me to essentially "pee" semen for 10-15 seconds. I believe in my experience this is a "true prostate orgasm". This is the most satisfying way for me to experience orgasm without the involvement of a second person.
I fuel the "fantasy in my head" by telling myself that I am just receiving a punishment enema for being a moody brat and that retention of the enema is mandatory for 10 minutes, regardless of whether an orgasm occurs or not. The fantasy is of course that I would have my moment and then be stuck, forced to continue to retain the churning soapy solution in my bowels until the timer sounded even though my enjoyment of the predicament would have been long passed. However, as soon as I've finished having an orgasm from enema retention I always disregard the remaining timer, remove the nozzle, and immediately sit on the toilet to expel. At least being in chastity ensures there is not an immediate release from that restriction because I intentionally leave the key in the other room.