Let's look at Linux for a moment. It's open-source and was originally supported by the community. Then came along companies like RedHat whose business model was to offer support for the free open-source linux. They don't sell Linux, but rather a business solution with full support. I know OpenG is at least an order or two of magnitude smaller than Linux, but that model looks like it could work. A company or contractor could very well offer paid-support for OpenG if an integrator wants to have fast turnaround time garantee on critical path tasks.
Whether or not this parallel stands further arguments, I don't think support response time is an appropriate metrics for classification. It will be part of the pros & cons of choosing to use OpenG or not, but as stated before, open source code means that you can hire anyone with a strong LabVIEW experience and he can fill this gap. That person could be an internal resource or an hired contractor... that is, if free LAVA support is not fast enough.