I think the tag order doesn't matter in XMI. In general, XMI just defines how to map MOF to XML. So basically you build the M2 model (=UML) from Java and and then map this to XML with XMI if I understand this correctly (and I think the double indirection is why you don't find good resources for the mapping from UML to XMI directly).
For many people, examples may be more accessible than the chain of transformation rules defined by the various OMG standards. So I'd just draw a minimal UML example diagram in my favorite UML tool for the export aspect I am interested in and look at the exported XMI. Then add features as needed...
P.S.: You may want to use the Java reflection classes (package java.lang.reflect) instead of parsing the the Java source yourself to generate XMI (if you don't need to preserve method argument names).