Is there any way to manually bless firmware when main disk is external?
To answer some of your questions:


The firmware is stored on main board. However, when updating firmware, the EFI system partition is used as a staging area.
After serval testings, it seems that Mac only recognizes the EFI partition at primary internal disk as a staging area (note: at least not working at the optical bay of my mac or at any external drive). So, if you only got an external disk or there is no ESP in your primary internal disk, that would be mission impossible, at least not the bless command could get it done.
To solve your issue, you may try the following steps if your internal disk has a EFI system partition:


boot the machine wish macOS
use bless command to prepare firmware update, including write scap file onto ESP.
make sure the scap file is on the ESP of internal disk. otherwise create one by copying it from ESP of external disk.
restart the machine. Hopefully, the upgrade will be processed.


or


backup your Windows
install High Sierra on the internal disk so that the firmware will be upgraded to latest one, or format the internal disk as GUID partition table (so that there will be a ESP on that disk for updating firmware), then use bless command to upgrade the firmware manually. note: strong recommend with --verbose and --recovery options.
restore Windows


or

I'm not sure is your mac be able to replace the internal drive with the external drive easily. If yes, I believe the fastest way is replacing the internal drive with your external drive temporarily to get the upgrade done.