I was thinking more about the request for the header button to remember it's last action (for that Sigil session), and I think it would be possible, but it would be a bit of an overhaul. The problem for me is not the ease or the difficulty of implementing it, but rather the change to existing behavior.
We'd need to change the current instant flyout behavior of the QToolButton to that of a delayed flyout. A long click to change (and fire) the new default behavior (with a mechanism to possibly change the icon of the QToolButton) and a short click to simply fire that current default action. That doesn't factor in how to get a configurable keyboard shortcut assigned to the current default action (nor how to only change the default behavior when the action is accessed via the delayed flyout).
I'm just not sure it's worth the trouble. It would definitely take more than 15 minutes (I'm not a professional programmer by any means 
), and delayed flyouts can be finicky, or downright annoying, for some users. Not to mention possibly confusing when the button changes something before they were ready for it.
EDIT: actually, this IS much simpler and less invasive than I originally thought (and no delayed flyout required). I'm not advocating one way or another here, but I think we can at least entertain objections (or votes of support) for such a thing.
The main issue for me would be that rather than needing to select an item from the popup menu, after an initial selection was made, merely clicking the headings toolbutton would initiate a change to highlighted markup. But the icon and tooltip would clearly indicate what the button's current action is going to be beforehand.