One way to do this is add a custom field to the Work Order (mrp.workorder) and create a Work Center called 'Signature' - then using the ID of this Work Center, conditionally show the custom field.

This way you can add this step to any routing that needs a signature and can have the signature requested at any point during the process.

Inherit the Work Order table view and use something like this:

<div attrs="{'invisible': [('workcenter_id','!=',3)]}">
  <center>
    <br/><br/><br/><br/><br/><br/>
    <h1>PLEASE SIGN OFF ON THIS WORK ORDER</h1>
    <br/><br/><br/><br/><br/><br/>
    <field name="x_sig" widget="signature"/>
    <br/><br/><br/><br/><br/><br/>
  </center>
</div>

This gives you something like this:

You may need to modify the spacing depending on the size of the device you normally use - a complete solution might use more dynamic spacing entirely.