Argh. The routines on the tile cutting page of the spreadsheet (1.0) don't work for me. I got output files, I got a little bit of imagery (right scene at more or less right location) but most of it (90%) was black. I used the OSGEO4W shell.
I've verified that I have all the .jp2 files I need (loaded into QGIS, they completely tile the area of interest).
I was hoping this was a cookbook method. I don't know nearly enough about these resources and tools to troubleshoot.
update: The purple shaded area is the size of the scenery. The tiny little smudge is the part of the .jp2 imagery that has been (successfully?) processed. I've overlaid this with my SRTM elevation data and as far as I can tell, the images are correctly located and scaled.