Health insurance waiting period - any point in even asking for it to be reduced?

It's too late now but what I'd have done in your situation (before the MRI scan) is to talk to the people in the Mater Private and ask them to see if the VHI would partially cover the MRI. You'd need to have had that approval sorted before you got the scan.

I can't see any insurance company reducing the waiting period. As they would see it, they only reason for asking is because you know you have some treatment or scan around the corner that you're going to claim for and that's why you're upgrading your policy. Which is precisely why the waiting periods are there in the first place.

The only time I've seen insurers waive waiting periods is as part of a lucrative company group scheme and, for group schemes in general, that's still the exception rather than the rule. Health insurance that only provides cover for public hospitals is not really worth the money you're paying because it is only covering the cost of a service you're entitled to anyway through the public system. It's not entirely useless as a product, some people get it as a cheap way to avoid age loadings when they turn 35. But at the very minimum you should aim for semi-private cover in a private hospital.