Magento – Dynamic code for logo src for use in email templates, possibly using short tags

For logo:

There is an email var for that:

{{skin url="images/logo_email.gif" _area='frontend'}}

To make it your own, upload and replace logo_email.gif

For email variables:

Some of these exist by default:

General Store Email address:

{{config path="trans_email/ident_general/email"}}

Store Name:

{{config path="general/store_information/name"}}

Store Code is trickier. This is not exposed in the config to my knowledge for use by the email templates. I suggest using a custom template file, and refer this way:

{{block type="core/template" template="custom/storecode.phtml"}}

storecode.phtml:

<?php

echo Mage::app()->getStore()->getCode();

Alternatively, and maybe less-desirable, are Custom Variables.

Custom Variables

System > Custom Variable > Add New Variable

To Implement a custom variable into a Transactional Email, do the following: In the Admin Panel, select System > Transactional Emails Click the row of predefined template to edit or click [Add New Template] button to create a new one. Create a new transactional email first if needed. In the Template Content field, select a location for your custom variable. Click [ Insert Variable ] and select the Custom Variable you would like to insert.

_{Source: http://www.magentocommerce.com/knowledge-base/entry/creating-custom-variables}

Without rewriting core classes you cannot pass variables to the e-mail templates. This is a real disappointment for me too. I needed this numerous times.
But here is a workaround.
Instead of having custom variables you can have custom {{...}} directives.

So instead of using {{var something}} you can use directly {{something}}.
But for this you need to create your own class that handles these directives (short codes).

Create your own class Namespace_Module_Model_Template_Filter that extends Mage_Core_Email_Template_Filter where you add this method:

public function somethingDirective($construction) {
    $params = $this->_getIncludeParameters($construction[2]); 
    //do something with params
    return expected result;
} 

$params is an array with what you pass to the directive {{something param1=value1 param2=value2....}}.

Now all you need is to tell Magento to use your template filter instead of the one it uses by default.

Unfortunately I didn't find a 100% rewrite-less method.
You still have to rewrite the email template filter model. But this should not be an issue if nothing else rewrites it.

You need to add this in your config.xml

<models>
    <core> 
        <rewrite>
            <email_template_filter>Namespace_Module_Email_Template_Filter</email_template_filter>
        </rewrite>
    </core>
</models>

I know this is not exactly what you were hoping for, but it provides a cleaner way of doing it than no rewrite every method that sends an e-mail. Doing this will make your new directives available in all templates.

Even if I say it a lot, and this may look like spam, I have to quote my sources.
This idea is not mine. I got it from Vinai Kopp's book Grokking Magento