There seems to be no direct solution.

What I did was this: 1. Edited the report in OpenOffice as far as that was possible. Doing this results in the sxw and rml beeing stored in the database in table "ir_act_report_xml", field "report_sxw_content_data" and "report_rml_content_data" with the id of the report you are working on, as binaries. Any changes can now only be made in the database, the rml file in the filesystem is ignored.

1. Download the rml content data to a file. Edit the fonts manually according to your specifications or change other stuff that couldn't be done with OpenOffice.

2. Update the according field in the database with the edited rml data.

If anyone has a better idea how to do this, I'd be pleased to hear.