The phase conductors would be sitting directly on the cable tray, a wire EGC will be no closer.
There are times where the tray is not suitable for use as the EGC because of the ampacity of the circuits, and there are cases with high ampacity circuits, tray 3' or wider, and a single EGC where this has been an issue. These cases are rare, and as I said before, I don't think there needs to be a code change...maybe an Informational Note, but it is something that the design engineer should think about if he/she is designing a circuit like this.
This is not something that you should be able to make a contractor change on his dime....A single EGC is code compliant, but in rare cases, is not an effective fault clearing path. This determination needs to be made by the PE who stamps the drawings and not by the EC who is installing the project.
This is very similar to the fact that some multi-duct duct banks with highly loaded circuits fail from excessive heat when installed per the rules in Chapter 3. The issue is addressed in Informative Annex B, but that is not an enforceable rule. It is not a real common issue, but it does happen, much like the issue being discussed in this thread.