If the user agent is a podcast app (should probably have some standardized UA for this), then you can just return the RSS in a 301 redirect.
This is how firebase app deeplinks work.
so if you want to make a QR code that works for everyone, maybe you could use Apple’s feed-based URL scheme and build a QR scanner into each podcast app that intercepts it. So iOS users that just use their a generic QR scanner get Apple Podcasts, and people scanning from their podcast app get a better experience.
The social network of the future: No ads, no corporate surveillance, ethical design, and decentralization! Own your data with Mastodon!