I would go with kml file (without compression).
You can write it, e.g. with MBS Plugin: Text.WriteTextFile
I would take an example kml file and use it as template.
Remove the lines for the entries.
Than add in a script the xml entries for each record.
On the end write them to file.
I do that in a solution myself.
I use the kml file format that Christian has suggested along with the Export Records script step:
First, setup your "file path/file_name.kml" in a variable like $filepath
Export Records[ No dialog ; $filepath ; Automatically open ; Unicode (UTF-8) )
using XML as the File Type.
You may need an XSLT file to convert the XML output from FMP to Google's KML format. Contact me by private message if you need an example.