uBlock origin exposes this via https://github.com/gorhill/uBlock/wiki/Dynamic-filtering:-ru... . In particular, https://github.com/uBlockOrigin/uAssets/blob/master/filters/... will translate requesting google-analytics.com/analytics.js into this stub: https://github.com/gorhill/uBlock/blob/master/src/web_access... .

Not sure if you can easily substitute arbitrary scripts (would probably be placing too much trust into filter lists) but the resource library seems to be quite extensive: https://github.com/gorhill/uBlock/wiki/Resources-Library#url...