I couldn't understand how to fill a slot without a product or anything. I understand that you can fill a slot from a template, but if Plone is not picking up that template, then the filling code would never be run. I ended up modifying main_template and putting my code directly in the . This is bad because different skins will have different main_templates and indeed it bit me because I've modified it for one template when I was using the other. That is not a harmless-nothing-happens experience but a nasty problem because main_template in on custom and it gets picked up so you have one skin working with the main_template of the other. End result: UI broken with a hard-to-find problem.
This is the code I added:
<head>
...
<link rel="openid.server" href="http://www.myopenid.com/server" />
<link rel="openid.delegate" href="http://pupeno.myopenid.com/" />
<link rel="openid2.local_id" href="http://pupeno.myopenid.com" />
<link rel="openid2.provider" href="http://www.myopenid.com/server" />
<meta http-equiv="X-XRDS-Location" content="http://www.myopenid.com/xrds?username=pupeno.myopenid.com" />
</head>
I will probably mark this answer as accepted because it's what I'm currently using (and that's my policy, I mark solutions I end up using as accepted, nothing else is marked as accepted), but if any of the other questions become clear in how to inject this new template, I will use that and revert the acceptance (if StackOverflow allows it).