Hi,
It's actually quite easy to do.
First, you'll have to create a new form control.
The control definition contains both the read and write views.
I suggest you base yours on the default text component and tweak a bit (add a javascript verification for the url format in edit mode. add a link tag in read mode)
Second, you'll have to update the form definition
This is configurable in share-config-custom.xml where you just redefine the form for your aspect.
Here you can override the default form control for your metadata and force it to use your custom control