I cannot find any workaround for the SOLO raw content post bug, http://www.openlaszlo.org/jira/browse/LPP-6683, so I am going to make the following slightly incompatible change in swf8 SOLO raw post behavior, in order to better support XML-RPC in SOLO mode.
In the case of a SOLO raw post, I am going to make the kernel assume that the raw content is valid XML, by calling the Flash XML.parseXML() on the content, and sending that via XML.sendAndLoad(). This is only a change for the swf8 runtime; DHTML and swf9 will support posting of arbitrary data content in SOLO raw post operations. If the content you are posting is valid XML, it will be posted verbatim as the POST data contents. If the content is not valid XML, it will get munged in whatever manner the Flash XML parser treats malformed strings when parsing them into XML. If you want to have the current behavior, you will need to XML-escape a raw string before sending it in swf8. That is my story, and I am sticking to it.
