if you need sda2 don't use 'usb' id, it is hardcoded to sda1 in linuxrc. In that case fill same fields like with any card partition and enable host mode (if you don't have cable that does it for you).

Most probably there are two issues left (did not try)

1. there may be missing /dev/sda2 device node in initfs

2. there is timing problem, usb storage device discovery is slow

1 can be solved by creating it by hand (in initfs_flasher/initfs/dev) from another shell while initfs_flasher is waiting for dropbear/bootmenu.conf answer

2 can be solved by hacking end of bootmenu.sh, there is case for special ids so you may need to add similar one to the end after line that sets usb mode to host. Simple sleep should be enough.