There is actually a patch for the firmware spring branch of uboot which introduces an increasing delay of up to 10 seconds. Someone mentioned it on IRC It is here: https://chromium-review.googlesource.com/#/c/65542/ I just rebuilt the nv_uboot_spring (which won't work for you) with said image. However, that doesn't help when you're trying nv_uboot from the slow device (and spring wouldn't help your snow anyway).
I would think the easy path would be to put in a SD card with the nv-boot image and have it boot, load the kernel from partition 2 of the SD card and then set the root filesystem to the /dev/sd(whatever the usb card is). You can set the $dev_extras variable to append a root=/dev/whatever to your boot process.