Is there an easy way to update a NetBoot image?
I think you've got the best way, I know you can mount the disk image inside the netboot image, but AFAIK it breaks the netboot; and some updates can't be applied to a non-boot-drive disk image. 

If you're netbooting a lot, I'd recommend using DeployStudio, a free, open-source powerhouse for network deployment and the like; it makes it a bit easier to update / deploy images than netboot - but it might be a but much for what you're using netboot for.