you're all right about the onions & peppers being banned due to their sgar content. Garlic has a similar amount but I know clients of mine have added it to soups (haven't asked about shakes but even the most bonkers ones wouldn't try this....would they..?) without problem.
The tiny amount of garlic needed to give a kick can't contain v much Carb can it. (but don't tell Camb HO I said it or I might get into trouble)