Sorry, but I don't follow the reasoning. Wouldn't proprietary projects be incompatible with the CC-BY-SA? Such projects would still have to pay you to use the font. I thought it was a "GPL-like" license for creative works.
As per licence text, CC-BY-SA only requires that you distribute your contributions to the material (so, in this case, font edits) under the same licence, not that you distribute your projects under that license. So that's LGPL rather than GPL.