I would like a more convenient method which doesn't involve editing a file and rebooting.
You can't. Well, that's not quite true; you could do it without rebooting but you would have to restart the GUI using a different configuration (see below).
Dynamic rotation in the GUI is normally handled by the Xorg server. Xorg uses a userland driver for the display stacked on top of the kernel one. This can be tailored to the hardware but in the case of the Raspberry Pi it currently is not (and perhaps there would be no advantage to doing so); a generic framebuffer driver,
fbdev, is used.
As a consequence of this, you are subject to the limitations of the
fbdev driver, which does not allow for dynamic rotation. It does, however, allow for static rotation as described here. This is a method distinct from using
config.txt; I would guess that using
config.txt would perform better because the rotation is performed in hardware, whereas with the Xorg methodology presumably it is not (but again, this is just a guess). However, as per that question some people have had issues with
display_rotate=3; other people have hinted this may be an easily correctable firmware issue. In any case you are free to try either one (trying both would rotate the screen several times).
If you want to keep an alternate configuration, create one as per the linked answer and put it in a new directory in
alt_config, call it
startx -- -config alt_config/rotate.conf
man startx and
man xorg for an explanation of how that works.