Tamil Brahmi Short Mid Vowels

[Richard Wordingham via Unicode]

A problem has been spotted with the rendering of Tamil Brahmi vowels -
in particular the sequence <U+11013 BRAHMI LETTER KA, U+11044 BRAHMI
VOWEL SIGN O, U+11046 BRAHMI VIRAMA> does not conform to the grammar
of the Universal Shaping Engine (USE); a dotted circle may be inserted
between the vowel and the pulli.

When considering font-level remedies, I realised that there may be a
problem with a following consonant - is <U+11013, U+11044, U+11046,
U+11022 BRAHMI LETTER TA> a correct encoding of what may be
transliterated as _kŏta_?

The nearest to a convincing justification I can find for it to require
U+200C ZWNJ after the virama is the text in TUS Section 12.1 for
*Explicit Virama*, but that merely says that ZWNJ is required to
produce explicit virama rather than a _conjunct_.  As I understand
it, a subscript final consonant would be encoded as consonant+virama
rather than virama+consonant, so there is no ambiguity in Brahmi text.
(If we try to make a rule out of two conflicting mechanisms, the
difference might be that one is used for viramas and the other is used
for invisible stackers, though that would require changing U+10A3F
KHAROSHTHI VIRAMA back to being a virama.) The problem is that a font
that tries to recover the situation might interpret <U+11013,
U+11044, U+25CC DOTTED CIRCLE, U+11046, U+11022> as having TA
subscripted to the dotted circle.  If ZWNJ is required for _kŏta_, what
text if any in TUS requires it?

Richard.