I would like to quickly append a note to the currently clocked in task - something like a capture. I don't believe there is anything out of the box to do this, but it seems it must be possible.
Any ideas how?
A capture template can have a target of
clock, which will file to the entry being clocked. Here's an example template that stores a note in a plain list item.
(add-to-list 'org-capture-templates '("C" "to clocked" item (clock) "- %?"))