Suppressing Ligation of Spacing Marks

[Richard Wordingham]

Should it be possible to suppress the ligation of a base character and
a visually following spacing mark in plain text?

The example I have in minf is the sequence <U+1A36 TAI THAM LETTER NA,
U+1A63 TAI THAM VOWEL SIGN AA>.  It may be desirable to suppress the
ligation because both ligands have subscript consonants.  However, if
I write <NA, ..., ZWNJ, SIGN AA, ...>, the Universal Shaping Engine
decides that the ZWNJ triggers a new syllable, and inserts a dotted
circle before SIGN AA.  (The dotted circle after SIGN AA results from a
failure to read the proposal for the Lanna script as it was then
called.)

Richard.