Ok, found a way to trick resume and have it wait for slow usb devices without a need of adding a particular mechanism into initramfs:
1. Comment the "resume device =" line in /etc/suspend.conf out
2. Run the script
3. After creation of the cpio archive uncomment above line again
4. s2disk (or s2both if preferred)
5. When booting the initramfs, resume will prompt for path to device, type
"/dev/sdXN", wait until device is ready and then hit "Enter"