Since 9.0+, FreeBSD has not been able to PXE boot directly from an ISO file. The kernel will start (having been set up by the loader), but multiuser will fail because the ISO image was only accessible in protected memory, before the kernel booted.
It should be copied as a linker file for the kernel to reference, and then geom should be modified to try to attach to linker files (at least this particular one), enabling iso9660 tasting to work and therefore mountroot.
I have done some work along these lines, but was not able to finish before switching to other work. If anyone wants to try to fix it this way (there may be other ways I'm not aware of), let me know and I will dig the patch out from the sands of time.
Improvements to this would be really helpful.