Yes, that works iff (1) you have writable media, (2) you can actually feed it to the kernel RNG (and have it credited) before anything critical blocks on RNG output, and (3) the seed isn't (intentionally or accidentally) predictable (e.g., cloning an existing filesystem image, or cloning a VM, or any other design where the same entropy file is reused by multiple instances).
The kernel has limited ability to protect against (3). Some forms of it can be prevented with the VM Generation Counter PCI device, which Linux recently added support for.
You don't actually have to have writable media on the system itself, as you should in principle be able to pass the entropy information to the system over netboot.
And now that I think about it, with BIOS being so sophisticated these days (with UEFI and the like), even before booting many systems should be able to use the network to get entropy information.
Regarding point (3), if an image is used in a VM, the VM host should be able to pass entropy to the guest, so the guest doesn't have to use its own entropy to boot.
The kernel has limited ability to protect against (3). Some forms of it can be prevented with the VM Generation Counter PCI device, which Linux recently added support for.