In reply to Bangla (Bengali) Font Discussion :
In other scripts I have put this looking in the 'ccmp' feature. Because ideally one wants to get the dotted circle out of the way before doing and text shaping. However, I don't think 'ccmp' is processed by Indic shaping engines (although you could try it). You could also try using the 'akhn' feature, which processes ligations before other shaping. But it might be that the Indic shaping engines prevent this kind of trick if the dotted circle is simply not passed to the shaping engine for OTL processing on the grounds that it isn't an Indian character.
As a last resort, you could replace the dotted circle glyph in the font with a zero-width no-outline glyph, still encoded as U+25CC. This should get inserted by Uniscribe instead of a visible dotted circle, solving your problem. Of course, the presence of such an invisible glyph in the string will still prevent complex interactions between glyphs on either side of it, but it should work for your aa-aa case.