I had this same question, and after a lot of research, it looks like it's not possible.
The answer from cgat is on the right track, but you can't actually concatenate references like that.
Here are things you can do with "variables" in YAML (which are officially called "node anchors" when you set them and "references" when you use them later):
default: &default_title This Post Has No Title
title: *default_title
{ or }
example_post: &example
title: My mom likes roosters
body: Seriously, she does. And I don't know when it started.
date: 8/18/2012
first_post: *example
second_post:
title: whatever, etc.
For more info, see this section of the wiki page about YAML: http://en.wikipedia.org/wiki/YAML#References
default: &DEFAULT
URL: stooges.com
throw_pies?: true
stooges: &stooge_list
larry: first_stooge
moe: second_stooge
curly: third_stooge
development:
<<: *DEFAULT
URL: stooges.local
stooges:
shemp: fourth_stooge
test:
<<: *DEFAULT
URL: test.stooges.qa
stooges:
<<: *stooge_list
shemp: fourth_stooge
This is taken directly from a great demo here: https://gist.github.com/bowsersenior/979804