Solution to the problem of mynahs usurping the nest logs intended for barbets and similar guests:

I have found the problem goes away if you provide at least 3 nesting sites.
I have one (wooden box) for the Mynas. Since they use it, they keep away all other Mynas, and I find are not too aggressive to other birds. This allows the Double-collared & Crested Barbets to have their own nests without disturbance. Extra nestboxes (of which I have two) have attracted Glossy Starlings (currently feeding their young) and Wrynecks.

Another advantage of this scheme is that you can keep an eye on the Myna's nest, and "manage" their eggs so that none hatch. It doesn't stop them staying at the same nest box, nor does it seem to make them more aggressive. This way, you can have plenty of hole-nesters.