I’ve never tried it or know anyone who has, but my first inclination would be to say this is not a good idea.
Let me explain a couple reasons for my concern over this.
Blood Sugar Instability Concern
Unless you are juicing only greens, the carb and insulin spike you may get from the juicing since you’re taking in the juice without the vegetable fiber could easily be too much for HCG purposes – if this messes with your blood sugar, creating highs and lows, this can hamper or prevent quality weight loss.
Protein Requirements on hCG
Even though the protein is not extremely high on this diet, the amount the original protocol dictates, 3.5 oz or 100g two times a day, seems to be very important for preventing excess muscle loss on such a low calorie diet. Dr. Simeon’s in his book outlining the original protocol made it clear that this was just enough to keep your body above protein deficiency while on the diet.
You may lose a lot more muscle and not fat without the meat proteins.
Even for vegetarians who do this protocol, they almost always include a more concentrated source of plant protein like a plant protein powder to compensate.
But you could certainly make some green smoothies- I have done this successfully- blendering up spinach with water and stevia is actually quite good!