Is there a way to return asset data along with entry data?
Yes - you can access it exactly the same way you do in the templates; you just need to use PHP syntax (e.g. -&gt; for accessing sub-properties rather than .).

'transformer' =&gt; function(EntryModel $entry) {
    // Begin defining the response data for this entry
    $data = [
        'excerpt' =&gt; $entry-&gt;excerpt,
    ];

    // Get the first asset in a "heroImage" Assets field (if there is one)
    $heroImage = $entry-&gt;heroImage-&gt;first();

    if ($heroImage) {
        $data['heroImage'] = $heroImage-&gt;url;
    }

    return $data;
})