Arthur --

For background, look at the PinPoint Reference, Scripting, Plate object, Plate.UseSExtractor property.

As far as I know SExtractor in its pure form does not do any catalog lookups. At least the one I ported to WIndows has no catalog I/O (and for which catalogs anyway?). It is a source extraction program period. I don't know what "the additional analysis parameter" or the "flags" are so I can't completely answer this.

Is there a simple way to tie an object in the sextractor output catalog to a GSC catalog ID other than doing a search for the nearest catalog star with a given RA,DEC?
I'm not sure what "simple" means, but your proposal to use NearestStar is how I would do it, using PinPoint to do the catalog I/O and (assuming the plate is solved) getting the X-Y to RA-Dec then feeding that into NearestStar, getting that PlateStar and using its IDentification.. PP will read in all of the cat stars in the image area, so this should be pretty fast.