Hello,
today I've tried the following:
Use .htaccess to rewrite the url conditionally if an image file was not found.
Rewrite target was http://www.this-is-the-live-system.com/site/assets/files/$1
I guess that would work out great. Unfortunately Processwire checks to see if the file exists and outputs an error message in the Page Editor.
An option to disable this check would be great. (Similar to $config->debugIf = '::1'; or something)
It would be great being able to just use the live database locally without broken images all over the place.
I guess this might be a simple good enough solution for most use cases.