Difference Solder Mask Expansion for Top and Bottom on a Via?

[sacherjj]

I want my vias on the component side almost tented, but open on the bottom.  This gives you the ability to capacitive test points for BGA to detect non-solder, good solder, and bridge for each via broke out ball. 

I have negative solder mask expansion, top small pad, bottom large pad.  I want a fully exposed annular ring on the bottom, but can't do that on the top without possible solder bridging on the BGA.

I can't find a way to specify solder mask extension on top and bottom, just one value.  Is the possible in Altium at all?

[sacherjj]

I might be doing something very wrong.  I'm getting polygons that are GND net, joining with every via under the sun.