Hmmm...turned off V18 434, fired up V17 (to remind myself what the image sequence render option is called), then reopening V18 434 and now image sequence rendering is there.
No filters, but I was masking away a logo using pan/crop.
This was the first time I'd run 434, so suspect it is some kind of initialization quirk. Maybe firing up V17 helped remind it of the other render options, so to speak.