Hello Garth,
I stumbled upon this very same problem a few years ago and couldn't find a direct way to solve this back then, and I think it's still not explicitly supported at the moment.
However, I found another way of providing extension specific resources by shipping a custom theme with the extensions with the resources. This gives you a predictable name to refer to resources.
However I think the resource lookup should be extended to also be able to lookup resources from the themes-resources folder of an extension jar. This would make it much easier to ship
self-contained extensions with UIs.
Cheers,
Thomas