I can confirm for XP, that "-context menu -open with -bluegriffon" opens BG, but not the file.
Also, if an instance of BG is already open, an additional instance will be created instead of opening an additional window in the existing instance.
IMHO, this is an inconvenience. For me, other issues would be more important, like destruction of PHP-statements or beeing able to insert something just outside of the current element without resorting to source view.