Anyone plagued by a slow dialup connection may find this line-art version of the schematic not as pretty as John's, but about 7 times quicker to obtain:

 

 

Fascinating project.

 

I would quibble a bit over the "fourth method" nomenclature in the VK3BHR article, though, since it still amounts to summing the output of two double-balanced modulators, each driven with I&Q versions of the audio and RF.  The only difference between this and other switched balanced modulators is, the RF drive to each switch-- comprising half of each respective DBM-- lasts for 90 degrees of the carrier rather than the usual 180.  You could replace the ring divider with the normal and inverted outputs of a standard quadrature divide-by-four circuit and achieve fundamentally the same result.

 

John D