Will try to reproduce asap.
Now my only issue is modifying the rss template.
Looks like this example is missing in our docs You can disable rss for a collection and define a custom route.
This should work (use the template name you want to choose):
routes:
/posts/rss/:
template: posts-rss
content_type: xml
collections:
/posts/:
permalink: /posts/{slug}/
rss: false
template:
- index