Some god awful combination in a complex filter using atrim to pull the pieces, adelay to set the positions in the output, and amix to put all of the output back together could probably do it. What that command may actually be is definitely open but that's probably the only way that would work as otherwise this is really 2 separate operations (chopping then merging) so there isn't going to be consideration for having it in a single premade flag.
On another note if the goal is just to avoid files/writing to disk then a bunch of ffmpeg splices to named pipes as inputs to another ffmpeg command to merge them could do the same without the command soup.