We are uploading source maps to Sentry but also host them publicly. Currently we are externalising some modules and we'd like to know if it's possible to use both at the same time.

You can use both methods simultaneously, these are not mutually exclusive.

We rely on source map scraping (searching for the hosted source maps) as a fallback when the files are not found in your project - either because you didn't try uploading them, or something unexpected happened during the source map upload.