Distinguishing COENG TA from COENG DA in Khmer script

[Richard Wordingham]

The modern Khmer language does not make use of a COENG DA distinct from
COENG TA.  The normal practice is to render them the same, with a
recommendation from Unicode that the choice be based on the sound the
subscript represents.  At least, there was such a recommendation; I
tried to find it again, but failed.  The visual distinction faded out
in the 1920's according to Antelme.

Now, the Khmer script is not just used for modern languages of
Cambodia.  It is used for transcribing Old Khmer (for words, at least)
and was the religious script of most of Thailand until the 19th
century, and was also the secular script in southern Thailand.  In
these usages, COENG TA and COENG DA are distinct, or at least, TA and DA
have distinct subscripts that are clearly associated with them.

Is it legitimate for a font to deliberately render the corresponding
named sequences differently while claiming to respect characters'
character identities?  I thought it obviously was, but I received a
demurral when I asked about the best way to request an arbitrary
OpenType font to make the distinction.  (I expect the overwhelming
majority would refuse to make the distinction.)  I am therefore asking
here for advice on the legitimacy of such a request. Conceivably we need
a new character to make the distinction.

Richard.