Currently we don’t know of an OPC server that reads from XML services, though one may exist.
However, this is also not the first time someone has posed this question to us. We have been considering a few options for working some solution into FactorySQL, but we haven’t made a decision yet.
Perhaps you could give us a little more information about how the service works, the format of the data, etc. Feel free to send it to support AT inductiveautomation.com if you don’t want to post it.
Also, just to clear up one thing, it’s not by chance using the OPC XML standard for the service is it? The opc foundation had defined a format for webservices, but I’m not sure that many vendors implemented it. However, a quick search showed that it may be more popular in building automation.