CesiumResource Proxy.getURL() async

I’m using the Proxy.getURL() method to sign the URLs for map tile requests for a CesiumResource. The map tiles live in a restricted bucket in Amazon S3, and my Proxy.getURL() method changes the request to a signed URL to satisfy the access control requirements for the bucket.

The Proxy.getURL() method must be synchronous, and the current S3 client only supports asynchronous usage (the AWS S3 v2 library allows for synchronous signing, but this is now deprecated).

https://cesium.com/learn/cesiumjs/ref-doc/Proxy.html

Is there a way around this issue, or does this need to become a feature request?

Thanks