We are also very interested in usage of UNII-3 channels as we struggle with channel interference in ultra high density environnements and also with DFS events.
I've already made search about this topic and regulations, as we have the same question.
Some EU countries appears to allow it with restrictions (max EIRP of 25mW) , and according to this article, it's the same case for Belgium & Netherlands, so if you comply with legal requirements, it should be OK to use these channels.
As my sites are located in France i've checked with the radio frequencies allowances published by the local regulator ANFR - they indicate also that this band is allowed for "non specific applications" - including wireless data transmissions with same requirements for power limitation (source).
Now, if you configured your Meraki network in EU domain, UNII-3 cannot be enabled on dashboard.
It's a shame as some other AP manufacturers allow this usage, and the hardware can support it...
Opened a case with Meraki , their answer was clear :
"Unfortunately, UNII-3 channels are currently not available in France (EU) regulatory domain networks, even when an EU country allows the usage of UNII-3 channels."