Can the default permissions on /dev/disk* be changed? Is it possibile to change default permissions /dev/disk*? 

In Linux this is done in the udev configuration files; which is the equivalent on OS X?

I'd like to give group write permissions so members of the group operator will be able to perform write operations to external hard drives without using sudo (e.g. run virtual machines from external usb drives).

I tried to sudo chmod g+w /dev/disk2* but it doesn't persist as when the first write attempt permissions revert to 640 instead of the changed permissions.
As far as I can tell, Apple locks this stuff down pretty hard to the point where it's not reasonably possible. 

However, considering what you want to do with it, I'm certain that what you're actually wanting is to give write access to the external HDD which can be found in /Volumes/ (after you plug it in). Now, most external HDDs I've used (that are formatted properly*) will allow anyone to write to 'em unless an Admin chmod 'em to say otherwise. 

*NTFS-formatted drives are read-only through the Mac OS. As far as I know, there is no way around this without reformatting the drive.