Looks like it was Wontfixed in Phacility because of complexity - https://secure.phabricator.com/T9608.

I guess any solution for generic-enough-template would be an XSS risk, but we already have a template solution for the URI itself, so maybe it won't be too big a risk to extend it to a body/header.
Evan also mentioned escaping - we'll need to escape the values differently if the content is JSON vs XML vs www-form-urlencoded.

It might be simpler to write extensions for each targeted build system.

(Not a solution)

At the moment I call an external script and I do whatever I want from there, using any languages of my choice (that is, PHP btw asdlol).