Instead of "proxyURL" lets add an optional "preprocessURL" callback function similar to the plugins that can transform the url. Then users can format the url however they need. Something like so:

if ( this.preprocessURL ) {

  url = this.preprocessURL( url );

}

managed this! for the capabilities loader, do you want a similar managemement? in my private example with credentials I am using the proxy url + the endpoint url of the wms capabilities in order to get the capabilities. but if you prefer I could add a preprocess url callback also for the capabilities loader

If there's no issue then I'd prefer not to add it since the user can construct it themselves before using the loader which otherwise isn't the case here. Is that right?

I think yes, it's fine 🚀

Since the options on imageSource are really the ones being used here it may make most sense to add getters / setters for those options instead of storing them multiple places:

get fetchOptions() { return this.imageSource.fetchOptions; }
set fetchOptions( v ) { this.imageSource.fetchOptions = v; }

Agree, but in order to make fetchOptions still usable as a parameter in the constructor from the client (and so for usage in R3F) I had to set the imageSource fetchOptions in the WMSTilesOverlay using the options, otherwise I can't set from ImageOverlay the fetch Options before the imageSource is defined.

export class WMSTilesOverlay extends ImageOverlay {

	constructor( options = {} ) {

		super( options );
		
		this.imageSource = new WMSImageSource( options );

		if (options.fetchOptions) {

			this.imageSource.fetchOptions = options.fetchOptions;

		}

		this.imageSource.fetchData = ( ...args ) => this.fetch( ...args );

	}
... rest of the code

if this the way to go I would consider implementing the same logic for all other overlays