This was bugging me too. I customized a field type and even made simple form types with a reduced fieldset (for more complex objects), but it just didn't feel right. I've come up with a solution that should do the trick :)
First - I think your <div id="team" data-prototype="..."> should be outside your loop.
Next, your prototype is just a form view object, so if you pass it to a template you can render fields by hand and customize the output. Note: autoescape has no effect on included content, so use 'filter escape'.
<div id="playerFields" data-prototype="{% filter escape %}{% include 'AcmeTeamBundle:Team:prototypePlayer.html.twig' with {'form': form.players.get('prototype')} %}{% endfilter %}">
Then in your prototypePlayer.html.twig, just render each field as you normally would in a form view.
<div>
<span class="title">{{ form_label(form.name) }}</span>
{{ form_row(form.name) }}
<span class="age">{{ form_label(form.age) }}</span>
{{ form_row(form.age) }}
{# render whatever else you like from your Form/PlayerType class... #}
<div>
My template has a different data structure to your question, but it should help :)
Cheers,
Chris
Deprecation Warning:
As of Symfony 2.1, .get() is deprecated and the above will not work. Replace form.players.get('prototype') with form.players.vars.prototype to fix this in later versions of Symfony.
form.players.get('prototype'). I'll update the post if it works.