merlyn and I have been doing some digging, which I'll record here in case this discussion resurfaces in the future.

I overgeneralized when claiming prior art in Smalltalk, and should have claimed prior art in VisualWorks, which was ParcPlace's "fork" of st80 (ParcPlace split off from Xerox Parc to commercialize Smalltalk). I have an image from the VisualWorks 2.5 era which contains an instance-side new, albeit in a dubious chunk of code that was later excised from the image. merlyn has verified that there are no instance-side new methods on VisualWorks 7.

So, the bottom line is that isn't good, solid prior art in Smalltalk for an instance new method. At best there was a failed experiment.