Currently, early microcode updates are supported by booting the kernel with two initrds: a microcode unage followed by booster's image.
But with 3.5M, the intel-ucode.img image is quite large. When not creating a universal image, booster could create its own early image with unneeded microcode code stripped. For my system, the stripped image is just 104k in size.

This can be done by running something equivalent to this command:
bsdtar -Oxf /boot/intel-ucode.img | iucode_tool -tb --scan-system --write-earlyfw=/tmp/iucode-stripped.img -

The final booster image that's created now just has to be appended as is to create a final bootable image.
Since the AMD ucode image is just 40K, stripping shouldn't be necessary and the image can just be prepended as-is.