I can't complete round trip ExportXML and ImportXML of a table that contains a "Large Number" (BigInt) field. Currently, the BigInt field is stored in the XML output with an attribute:
<xsd:restriction base="xsd:decimal">
When the XML that was exported from Access is imported back in to Access the field type is changed to "Number" and then all values that are bigger than Number fail to import.

When you say that you support BigInt I expect built-in functions to support it. You can test this using the export and import buttons in the GUI.