Enabling Digital Signatures in InfoPath forms created from pre-defined schemas

Recently I was working on some InfoPath samples for United Nation's Electronic Documents standards (UNeDOCs: https://www.unece.org/etrades/unedocs/), and one of the key requirements I am looking at with InfoPath is the ability to digitally sign the form. Sounds like a no-brainer, since this feature is given with InfoPath.

However, after I happily imported the schema into InfoPath and attempted to enable digital signature:

Ok, so InfoPath didn't allow me to enable digital signatures. What's wrong with my xsd?
A little click on the least-clicked button in InfoPath (that's the little [?] help button) and a quick search on "Digital Signature" reveals:

Take note of the... well, NOTE. "If a form template was designed based on an XML Schema, you can enable digital signatures for the form template only if the XML Schema has a node that is in the W3C Xml digital signature namespace". Well, that's compliance for you.

So, using the all-powerfull killer app (that's NOTEPAD.exe), I open the xsd file and added a new element for my digital signature and point it to the W3C digital signature namespace: https://www.w3.org/2000/09/xmldsig#
as such:

<xsd:element name="DigitalSignature">
    <xsd:any minOccurs="0" maxOccurs="unbounded" namespace="https://www.w3.org/2000/09/xmldsig#" processContents="lax"/>    

Source: https://www.w3.org/TR/xmldsig-core/#sec-Versions

Voila, now InfoPath allows digital signatures on my form!