Oh man, that would be nice. If I may vent for a moment, the way that the WP devs chose to implement that excerpt function is absolutely idiotic. Grade F bullarky stuff (bring on the flames, the source code tells all). So trying to tie into that function and offer it inside of summarize-posts was an exercise in futility and madness that I do not care to repeat. So much of the WP core is coded very poorly, I'm afraid, so using its functions outside of the narrow scope in which they were conceived can be incredibly difficult or impossible. the_excerpt() the_content() and all of that are perfect examples of how NOT to code something (in my biased opinion). The insane devotion to global variables, the over-reliance and pollution of events, and the consistent in-line PRINTING of values instead of the more sensible RETURNING of values is really a recipe for disaster, as you'll quickly discover if you try to customize and scale any WordPress site. It really drives me nuts that WP has this clean and appealing face, but this rotted nasty stuff behind the scenes.
The easy solution here? Add the excerpt manually. The more flexible solution? Use an output filter (e.g. a custom one) to come up with your own variation of the "excerpt" functionality that works cleanly.