When is ValidateValueCallback planned to be implemented? Are there now alternative solutions for custom elements (without binding to VM)?

We got a request about this in the past but it is not implemented yet. I've pushed the ticket #1512 to the next 3.2.3 version.

In the meantime the only alternative is to use the CoerceValueCallback, although it is not the same behavior, it can produce values that are valid for your control.