A request to https://meta.wikimedia.beta.wmflabs.org/static/current/extensions/MobileApp/config/android.json returns a 403 (Forbidden) response. Is this intentional? It would be handy to be able to test remote config changes on the beta cluster before they hit production.
That is defined in operations/mediawiki-config.
static/current/extensions is a symbolic link to /srv/mediawiki/php/extensions
/srv/mediawiki/php is mediawiki-config symbolic link /php. It points to the current version eg php-1.29.0-wmf.19 when beta uses php-master.
Extensions assets path is $wgExtensionAssetsPath = /w/extensions (forged from $wgResourceBasePath = '/w').
So instead of /static I think you should use /w/extensions. That is rewritten by Apache to point to /w/static.php which does the routing.
All the details are on the rather long T99096: Make Varnish cache for /static/$wmfbranch/ expire when resources change within branch lifetime