What an interesting dream! But sadly, it must remain a dream. Firstly, 2C-B and mescaline is not only separated by the bromine at the 4 position compared the oxygen at the same position with mescaline. The position of another oxygen is also a difference, at position 3 for the mescaline and position 2 for 2C-B.
Mescaline is 3,4,5-trimethoxyphenethylamine, while 2C-B is 2,5-dimethoxy-4-bromophenethylamine. Following your dream hypothesis one would end up with 3,5-dimethoxy-4-bromophenethylamine. That's an unknown compound, but I guess it would be psychoactive, although likely with lower potency than 2C-B on par with mescaline.
There's more problems though. It simply will not do to add bromine. The reason mushrooms can produce exotic compounds such as 4-hydroxy-DPT or 4-hydroxy-DIPT by adding DPT or DIPT is that the enzyme that 4-hydroxylates dimethyltryptamine into psilocin (4-hydroxy-dimethyltryptamine) doesn't care too much about its substrate. Next to any tryptamine compound can therefore be 4-hydroxylated by the mushroom.
Cacti doesn't have this 4-hydroxylating enzyme. The biosynthesis of mescaline goes as this:
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Now, as you see already in the first step where phenylalanine is hydroxylated into tyrosine, the 4 position is occupied by an oxygen rather than a bromine. At the last step of the biosynthesis this position undergoes another change, by O-methyl transferase the 4 position now carries a methoxy group.
In order for your idea to work, you would have to genetically engineer the cactus to produce a phenylalanine brominating enzyme rather than an phenylalanine hydroxylase. You would also need to disable the last step of the O-methyl transferase, not to mention make sure that all other enzymes at play don't care if their substrate have a huge bromine stuck to it.
That way you would end up with 3,5-dimethoxy-4-bromophenethylamine. However, if the end product rather should be 2C-B (2,5-dimethoxy-4-bromophenethylamine), you would also need to engineer the cactus to hydroxylase and then O-methyl transferase the 2 position instead of the 3 position. Put simply, this is all extremely difficult. It can't be done by adding anything to the cactus.
Nice dream though!
