You would need to do a look up on the Products in the cart and see how many, if any, have a weight > 70 lbs ...

There is a nice little function in the shopping_cart class to test various conditions of the products table ...

function in_cart_check($check_what, $check_value='1') {
Then you would need a method to run that function on decide whether or not to allow the shipping module to show ...

It is usually best to make the test a function that can then be used to manage both the admin and catalog as the admin needs to call the function and just return true ... where the catalog you may have other checks or tests to run ...

You are more or less trying to control the $this->enabled in the shipping module ... when false, that hides the shipping module even if the shipping module is installed ...

Easy Smeazy eh?