Assuming the ZL600 discs are the same as the ZL900/1K (i.e. not floating or semi floating) any competent machine shop should be quite capable of doing the job for you, so long as they run true when completed.
I would only machine the active portion of the disc though (the section the pads run on). To resurface the whole face on both sides of the disc will reduce its rigidity. As these are not floating discs they need to remain pretty solid on their mounts to ensure smooth brake operation.
Most machine shops should be capable of advising up front if machining will exceed the minimum thickness limits as marked on the disc. If you find they need to be replaced, they may even be able to manufacture replacements of your design. These are fairly simple to copy; you only need be sure the material used is suitable.
Let us know how you get on and good luck…