The AzureEventProvider is a public class and quite a few of the methods of overridable. As that value is set in the constructor you could create a custom version inheriting it and inject it in as the default.

The problem is that SubscriptionName only has a Getter, which uses the private variable _uniqueName which is set in the DefaultServiceBusEventProvider constructor. 
So there is no way to set _uniqueName without duplicating DefaultServiceBusEventProvider or maybe using Reflection to set _uniqueName in a inherited class.

I don't see any code relating to DefaultServiceBusEventProvider in any projects I've got on DXP/Azure.

There's an AzureEventProvider which you mentioned in your post and the one I was looking at which has

    public AzureEventProvider(
      AzureEventClientFactory clientFactory,
      EventsServiceKnownTypesLookup knownTypesLookup,
      IServiceBusSetup setup)
    {
      this._clientFactory = clientFactory;
      this._knownTypesLookup = knownTypesLookup;
      this._uniqueName = Environment.MachineName.Replace('/', '_').Replace(':', '_') + Guid.NewGuid().ToString("N");
      this._serviceBusSetup = setup;
    }

To which as this is a public non sealed class I woud just

public CustomAzureEventProvider : AzureEventProvider
{
    public AzureEventProvider(AzureEventClientFactory clientFactory,
      EventsServiceKnownTypesLookup knownTypesLookup,
      IServiceBusSetup setup)
{
      this._clientFactory = clientFactory;
      this._knownTypesLookup = knownTypesLookup;
      this._uniqueName = WHATEVER YOU LIKE
      this._serviceBusSetup = setup;
}
}

Then just configure structuremap to use this instead

This actually might be a bug that needs to be fixed by Opti. What if I take my site and host it in AKS (for example). They have pretty long pod names that sometimes exceed 50 chars.

I have created an implementation over AzureServiceBusProiver which resolves this issue by using reflection. Actually the way i found this bug was when using Kubernetes and a Pod in a Deployment, which gets a hash suffix appended to its MachineName. I have created a issue in Customer suggestions aswell, so hopefully Optimizely will see the error and fix it
 
The implementation can be found here along with some self promotion to my post 😁 https://dev.to/stoffe/optimizely-on-kubernetes-4k9n