Index variables in Replace/Insert/Delete: Bug or not a bug?

At my current customer I'm to process Attachments in Oracle Service Bus (11g).
I get a soap message, in which several documents are registered to be processed in a Content Server.
For each document the content is delivered as an soap/mime-attachment.

Because of some requirements I need to store the message, complete with the attachments Base64 encoded, in the database. So I have to pick each attachment, base64 encode it and then insert the content at the corresponding document in the soap message. So I need to do an insert or replace of a specific element of the body variable based on an index variable.

It turns out that you can perfectly do an assign with an expression like:

$body/stag:StageDocumentsRequestMessage/Payload/stag:documents/stag:document[contentId/text()=$contentId]

to a variable, for instance called document.

I can do an insert of the base64-encoded content into that document variable. But that does not get into the body variable. Since, apparently, document is a copy of and not a reference to the particular node.

So lets do a replace with the xpath-expresin:

$body/stag:StageDocumentsRequestMessage/Payload/stag:documents/stag:document[contentId/text()=$contentId]

in the variable body. But this gives the error:

[PL_MyPipeLine, Request Pipeline, HandleAttachments, Delete action] XPath expression validation failed: An error was reported compiling the XPath expression: XQuery exception: line 34, column 91: {err}XP0008 [{bea-err}XP0008a]: Variable "$contentId" used but not declared for expression: declare namespace stag = 'http://www.darwin-it.nl/CS/StageDoc';
declare namespace jca = 'http://www.bea.com/wli/sb/transports/jca';
declare namespace wsp = 'http://schemas.xmlsoap.org/ws/2004/09/policy';
...

Same counts for Insert and delete: I thought of inserting a new version of the node in the list and delete the old one, but that would not work either.

I've googled around, and found several occurences of basically the same problem, but no satisfying solution.

At support.oracle.com I found the following bug:
"Bug 17940786 : CANNOT USE INDEX VARIABLE IN THE REPLACE ACTION WITHIN FOR-EACH LOOP" with the following description:

The customer uses 2 for-each loops with index variables ($i, $j).
In the Replace action, in the Xpath expression buider, they want to use
"./entity1[$i]/entity2[$j]". This is not permitted by the editor. The problem
also occurs with only 1 variable like "./entity1[$i]/entity2[1]".

However, for no apparent reason, this "bug" has the status "92 - Closed, Not a Bug". So apparently, Oracle finds it as "functioning as designed". But why can't I modify or delete a particular node indexed by a variable?
Apparently I'm now stuck with building the document list document-by-document and do a replace of the complete document-list...