I have written an entire API set for Salesforce.com inside of PO and it has been quite successful.
Why would you not want to use the Web HTTP request? It is faster and better to use in the salesforce.com case. (like 30 seconds per call faster)
Your output format should be XML and your header should be "text/xml;charset=UTF-8".
You should have an additional header of SOAPAction = "" as well
I would be happy to share the TAP with you if you would like. It is a community TAP so there is no warranty/support with it.
It currently includes the following functions:
1) Login
2) Create Case
3) Create Case Comment
4) Create Email
5) Get Updated
6) Query
7) Retrieve
8) Send Single Email
9) Update Case
So it has most of the core/basic functions and you can edit those to your liking. To answer your original question, unless it's defined in the WSDL it would not work in the web service execute, another reason to use Web HTTP Request. I have spent about 6+ months working PO with the SF.com API so I know it pretty well.
If you wish to get the TAP, you can email me @ shaurobe@cisco.com
--Shaun Roberts
Principal Engineer, CX
shaurobe@cisco.com