How to lookup a snippet by key?

[garyoberbrunner-gpsw]

I'm writing some code to auto insert a snippet into files by major-mode. I have an alist for major-mode -> snippet name, but it seems like the snippet keys are a more straightforward way to identify a snippet... except I can't find a function to lookup a snippet by key. Is there such a thing, something like yas-lookup-snippet?
Also if someone's already done this, feel free to point me in that direction.

FYI, here's my current code:

;; Define an alist mapping file extensions to yasnippet names (*not* keys!)
(defvar my-extension-snippet-alist
  '(("cpp" . "C++ CPP source file header")
    ("cxx" . "C++ CPP source file header")
    ("h" . "C++ CPP header file header")
    ("hpp" . "C++ CPP header file header")
    ("hxx" . "C++ CPP header file header"))
  "Alist mapping file extensions to yasnippet names.")

(defun my-auto-insert-snippet ()
  "Automatically insert a specific yasnippet when creating a new file based on its extension."
  (when (and buffer-file-name
             (zerop (buffer-size)))
    (let* ((file-extension (file-name-extension buffer-file-name))
           (snippet-name (assoc-default file-extension my-extension-snippet-alist)))
      (condition-case err
          (when snippet-name
            (yas-expand-snippet (yas-lookup-snippet snippet-name))) ;;; here's where I'd like to look up by key instead
        (error
         ;; maybe no such snippet
         (message (error-message-string err))
         nil)
        ))))

(add-hook 'find-file-hook 'my-auto-insert-snippet)