Is is possible to modprobe a module from the boot loader?

To load a module when the kernel is called from the bootloader you have only to add the name of the module.

The syntax is the name of the module, if you want to pass an option to the module, type modulename.moduleoption.

For example, in my old computer I have an ISA Plug & Play Ethernet Adapter RTL8019/11901 that works with the NE module. On the kernel command line I add ne.irq=0x0220 and boot. Finally the card is recognized.

Grub uses the insmod command to load up modules. Here's a real-world example from my /boot/grub/grub.cfg file:

menuentry 'Ubuntu, with Linux 3.2.0-23-generic' --class ubuntu --class gnu-linux --class gnu --class os {
    recordfail
    gfxmode $linux_gfx_mode
    insmod gzio
    insmod part_msdos
    insmod ext2
    set root='(hd8,msdos1)'
    search --no-floppy --fs-uuid --set=root 1b66bb9e-5b02-49f1-8cf9-bc3f649d70a6
    linux   /boot/vmlinuz-3.2.0-23-generic root=UUID=1b66bb9e-5b02-49f1-8cf9-bc3f649d70a6 ro   nomodeset pcie_aspm=force
    initrd  /boot/initrd.img-3.2.0-23-generic
}

Whether or not you can script that from yaboot, is another question. If it does work, please leave a comment saying so.

Add the module to initrd. There is thread on ServerFault. Refer to the answer that starts with "On a Debian system, you edit /etc/initramfs-tools/modules, and put one mo..."

Are you sure you can't just add your module to /etc/modules? That is the easiest way to go.