Is there an easy way to update a NetBoot image? I have a NetBoot image which I use for booting into to do things like disk maintenance and testing new applications without having to install them on my workstation. It's really handy, but I created it a while ago and much of the software on it is now out of date, like the OS, QuickTime, and Safari. I want to update the NetBoot image but the only way I think to do that is to use Disk Utility.app to restore it to a USB drive, boot from the drive directly, make any changes necessary and re-image the USB drive, overwriting the original image.

Is that the best series of steps or is there an easier way to update a NetBoot DMG file?
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.