Okay, so, I talked to Dan about this and drew up this mini shellout usage guide from the conversation:

https://github.com/opscode/chef/commit/74d161df2e6e24a4126962ef5b70e7d08e1e39e0

So it is cool to extend Chef::Mixin::ShellOut directly here, but be sure to be consistent in your use of shell_out versus shell_out! below.