According to the docs at http://www.microsoft.com/typography/otfntdev/indicot/other.htm, uniscribe renders combining marks in isolation when they are applied to SPACE + ZWJ. (Without the ZWJ, it uses a dotted circle.) Perhaps this is an acceptable solution to the people calling for a new character.
Combining marks and signs that appear in text not in conjunction with a valid consonant base are considered invalid. Uniscribe displays these marks using the fallback rendering mechanism defined in the Unicode Standard (section 5.12, 'Rendering Non-Spacing Marks' of the Unicode Standard 3.1), i.e. positioned on a dotted circle. Please note that to render a sign standalone (in apparent isolation from any base) one should apply it on a space (see section 2.5 'Combining Marks' of the Unicode Standard). Uniscribe requires a ZWJ to be placed between the space and a mark for them to combine into a standalone sign. Noah
