I'm very interested in this as well - how have you got on with extending the product class - is it feasible?

To add new attributes/fields to my products I added all required fields to the Product.php file and created php and ss files for each product type, that only showed the relevant fields.

So to add a product type called MyProduct with a Width attribute:

1. Add Width to ecommerce/code/Product.php:

class Product extends Page {
public static $db = array(
'Price' => 'Currency',
'Weight' => 'Decimal(9,2)',
'Model' => 'Varchar',
'FeaturedProduct' => 'Boolean',
'AllowPurchase' => 'Boolean',
'InternalItemID' => 'Varchar(30)',
"Width" => "Decimal(9,2)",
);
}

2. Create ecommerce/code/MyProduct.php:
<?php
class MyProduct extends Product {

static $add_action = 'MyProduct';

function getCMSFields() {
$fields = parent::getCMSFields();
$fields->addFieldToTab("Root.Content.Main", new TextField("Price", "Price"));
$fields->addFieldToTab("Root.Content.Main", new TextField("ShortDescription", "Description"));
$fields->addFieldToTab("Root.Content.Main", new TextField("ArtNr","Article number"));
$fields->addFieldToTab("Root.Content.Main", new TextField("Weight", "Weight (kg)"));
$fields->addFieldToTab("Root.Content.Main", new TextField("Width","Width"));

// product image field
if(!$fields->dataFieldByName("Image")) {
$fields->addFieldToTab("Root.Content.Images", new ImageField("Image", "Product Image"));
}

// flags for this product which affect it's behaviour on the site
$fields->addFieldToTab("Root.Content.Main", new CheckboxField("FeaturedProduct", "Featured Product"));
$fields->addFieldToTab("Root.Content.Main", new CheckboxField("AllowPurchase", "Allow product to be purchased",1));
return $fields;
}
}

class MyProduct_Controller extends Product_Controller {
}
?>

3. Create ecommerce/templates/Layout/MyProduct.ss as you would any other template