ive been wanting to try this, but dont have access to a good juicer.
alternatively one could blend up the chuncks really well, then place in the fridge for a day or so to let the foam settle and the pulp seaparate from the juices and float to the top,. which could then be simmered in water to recover whatever alkaloids may be left over, like you mentioned.
i wonder if anyone has tried either method? the juicer method seems ideal for preparing fresh cacti. drying and making tea, is a bit of a chore. i dont think it would be necesary to extract from the pulp really. nearly all alk's would be in the juice i would assume.