[frontendbeauty]

(OP here) Probably your best bet is to create a small Node web service that takes a template name and data and returns HTML. Then your (Rails, Python, PHP) app can call out to that in the request cycle.

Instagram.com does something similar; it's a Django app.

[icebraining]

Seems to me that the approach with best separation of concerns would be to redirect the initials requests directly to node, which would then call the Python/Ruby/etc API to get the data to render the template. After that, the browser JS could call the API directly, and so the latter would never have to know about templates at all.