Update: A Tamil Wikipedian, Mahir, went to the core of the issue that I cited in my previous email and identified that the issue in that instance was due to the superfluous use of the zero width non-joiner HTML entity. We're going to file a bug asking Mediwiki to chomp those entities when they occur in inappropriate places.
Qn: Definition for "inappropriate places"? Ans: Wikipedia URLs should be considered as "identifiers" and should use Unicode standard for Identifier definition using unicode data. Unicode Standard Annex #31 defines this clearly. http://unicode.org/reports/tr31/ IMHO, Mediawiki should implement this standard.
But for Tamil, I am not aware of any valid pattern where ZWJ or ZWNJ is valid. I am aware of valid patterns for other Indian languages. So in that case we should remove all zwj,zwnj from Tamil urls. Sometime back, the inbuilt tool in Malayalam wiki used to allow putting n number of zwj in text and we corrected the script to disallow user to put more than one zwj, zwnj in sequence(this is what UAX #31 says too).
Thanks Santhosh