Maybe you could try "consuming" only 250ml (just to fill the rectum or sigmoid/descending colon) each time, which may then be less likely to reach the systemic circulation.
I used to do coffee enemas and I always just consumed enough to fill the bottom part of the colon.
The hepatic portal system takes the coffee straight to the liver, which then goes to the system circulation, so maybe if you just dose enough for the liver to use/metabolise for itself, the effect won't spill over too much.
Another thing you can research is cholegogues (bile dumping stimulators), which you can make into a tea for an enema. One example would be yellow dock. I used to use a homemade formula for bile ejection when I was doing Liver Flushes in the evening. Of course, coffee would have kept me awake with an enema at ~22:00, so I'd do an enema with yellow dock or maybe dandelion root.
I'd read that caffeine is required for the effect of a coffee enema, although really it's the bitterness (like in all "bitters") that stimulates bile production. That's why people take digestive "bitters" to improve digestion (bile flow).
Maybe you could consume a good amount of fat and lemon juice just before the coffee enema to potentiate the effect of detoxification/flushing (e.g. 1 tablespoon of olive oil and lemon juice every hour before the enema - up to a few hours).
I know you're pretty creative, so I'm sure you can let your imagination take you to some interesting places...