Theoretically, you should be able to use a string such as
Space-ZWJ-<combining character> to get a combining character in
isolation. However, for Indic scripts, this can fail, because there is
not always a simple one-to-one mapping from combining character input
to final composed output.
You may be able to select the glyph directly by the glyph ID in the
font. Or, you could do something like
base+combining\llap{\textcolor{white}{base}}.
Hope this helps,
Andy
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