Hello Ant.  You can retrieve the catchall name by calling GetRequestedDomainSettings.  In the requestedSettings string, use 'catchall', and the name of the domains catchall will be returned.  I hope this helps.

 

POST /Services/svcDomainAdmin.asmx HTTP/1.1
Host: mail.exampledomain.com
Content-Type: text/xml; charset=utf-8
Content-Length: length
SOAPAction: "http://tempuri.org/GetRequestedDomainSettings"

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <GetRequestedDomainSettings xmlns="http://tempuri.org/">
      <AuthUserName>admin</AuthUserName>
      <AuthPassword>password</AuthPassword>
      <DomainName>example.com</DomainName>
      <requestedSettings>
        <string>catchall</string>
      </requestedSettings>
    </GetRequestedDomainSettings>
  </soap:Body>
</soap:Envelope>