The straight answer is that it doesn't work.
The only workaround I've used befor is having a shared network folder that both your user and the user running the SAP dialog service have access to. This does work, but you could argue it is a little messy.
That windows user is probably named SAPservice<SID> on the OS level (or equivalent for Linux).
Windows Operating System User Settings in an SAP System - SAP NetWeaver Security Guide - SAP Library
Share a folder and grant both acess. Afterwards you can run the bapi with the network path.
espen