So the basic issue with WebP is, that it generates .htaccess files in subdirectories thus overriding WPPPs rewrite rules. So accessing a not existing image size no longer redirects to WPPPs dynamic image creation routine.
Version 2.2.3 includes the option to add “RewriteOptions InheritDownBefore” to the rewrite rules wich basically forces all .htaccess files in subdirectories to also use WPPPs rewrite rules. So when enabled the first access to an intermediate image size, that doesn’t exist, will be redirected to WPPP and all subsequent request will be handled by WebP Express.
You can’t use WPPPs option to save intermediate images into a different folder than the original, because then WebP Express won’t find the original.
BTW: Now that I’ve seen how its done, I’m considering to add creation of intermediate images in WebP format directly into WPPP, including dynamic redirection for browsers not supporting WebP. Kind of a “WebP Express Lite”, as I would only integrate conversion using GD or Imagick.