Everything that I found out about boron suggests it has an LD50 similar to that of common salt.
I personally dont consume too much as I always tend to er on the side of caution and just include it with weekly intake of vitamins and minerals.
I do seem to remember coming across tamarind before but as always lose the link or forget to earmark it.
I always use tamarind paste (54% tamarind) in my stir fry's which adds to the flavour and as you say has added benefit of increasing excretion of Fluorides. Im not sure how much of the useful component in tamarind is destroyed during the cooking process but I do intend to get organic paste in the future with higher content or just maybe make my own.
anything that increases the removal of fluorides is a good thing without doubt - thanks for the post - good information.