I came up with an alternative for the fact that the cluster bounds property is not writeable. It came out quite elegant. Hope it's what you are looking for.
I am adding this to my user library as a merge vi, so the refnum, subvi and cluster with two string controls are always together. Very useful. If you make sure that the string controls are in position 1 and 2, you can omit the piece of code that searches the correct refnums (the while loopin the subvi). You can also replace the invisible controls with controls you are actually using in your cluster.
