With v0.2.x you can create a file STARTUP_RECOVERY on the root folder of a usb flash drive. The file should be empty (blank).
It is supported in the root of a device without a partition (floppy disk mode) or the root of the first partition.
The kexec-multiboot scans for sda, sda1, sdb, sdb1, sdc, scd1, sdd, sdd1. if you have more than 1 device connected.
The file needs to be uppercase and double check that windows doesn't append the .txt extension or something similar.
If you need to switch to the slot0 just plug in the usb stick, reboot or power off, and finally remove the stick after the reboot is complete.