To add a new field to a form in XML you just type:

    <record model="ir.ui.view" id="sale_margin_percent_3">
        <field name="name">sale.order.line.tree.margin.view.form</field>
        <field name="model">sale.order</field>
        <field name="inherit_id" ref="sale.view_order_form"/>
        <field name="arch" type="xml">
           <xpath expr="//field[@name='order_line']/tree//field[@name='price_unit']" position="after">
                <field name="sale_margin_percent" groups="base.group_user"/>
            </xpath>
        </field>
    </record> 

which would add "sale_margin_percent" field after "price_unit" in "order_line".

So far, so good.

However, what would you need to add to specify that the new field has a given "on_change" attribute?