How can I get to recovery without Volume Up hardware button? Or, 'My tablet's hardware "Volume Up" button is broken, now what?'

I'm trying to root an Asus TF300TG Transformer tablet (a 2012 model, unlocked), following this guide: http://forum.xda-developers.com/showthread.php?t=2617298


I have installed the USB drivers on my computer
Rebooted tablet to fastboot
Flashed the TWRP recovery without problems 
(Rebooted normally, verified that tablet still works as usual)
Rebooted to fastboot again
Got to the point where I should boot to the new recovery


That step requires me to press the Volume Up button to select the recovery option (at this screen: http://i.imgur.com/Fbogr.jpg ). In my tablet, however, the button is broken (and has been for some years now). Is there a way to get into recovery without actually pushing it?

What I've tried:


pushing harder
pushing the other two HW buttons: Volume Down moves the selection, Power does nothing
connecting the keyboard dock: there is a Volume Up button, but it doesn't do anything in fastboot mode; none of the dock buttons do anything
connecting a USB keyboard via the dock: no keypresses seem to do anything in fastboot
rebooting to recovery using fastboot reboot recovery: invalid command
booting an image using fastboot boot twrp.blob (and various other recovery images from my computer): loaded to tablet, then failed (downloading 'boot.img'...
OKAY [  0.398s]
booting... FAILED (remote: ) - note the empty error message)


Is there a different way to get to recovery?
Yes, there is a way.


boot the tablet normally
enable USB debugging
connect to computer by cable
from the computer, run adb reboot recovery
tablet reboots directly to recovery
Done!


I was trying to issue this command in Fastboot (where it's not supported) instead of ADB (where it works).