I have more or less same setup and problem.
SAP HR system language = Dutch
IDM SAP driver set to UTF-8
Remote loader on SAP server (SLES10)
Driver/Engine on SLES9.
We've seen the problem that when an iDoc segment conains a diacritic like ö
(O Umlaut), this string is not send to the driver by the remote loader.
You can compare the iDoc (using the sap viewer) with the iDoc file in wich
there characters get translated wrongly.
Im my situation a modify was done on an existing object so this attribute
now had two values. One with begin and enddate in the past and the one with
the diacritic with "valid" timestamps. The strange thing happened that the
old value was send to the driver!
I have an open incident for it and will update this post with my findings
But if anyone has the right setup for this, please don't hold back and
enlighten us :-)