good news, I got it working today

The issue is PX_Save is sending ComDoPropExchange with an object of the wrong class as parameter (cSigCJPropEx_Mixin.pkg):

            Get ComCreateAsRegistry of ghoSigCjPropEx False ("HKEY_CURRENT_USER\" - sRegKey) to bOk
            If bOk Begin
                Send PX_Delete_Key sRegKey
                Send ComDoPropExchange (pvComObject(ghoSigCjPropEx))
looks like ghoSigCjPropEx can't be used for ComDoPropExchange because of it's base class (cSigCjComSuiteControlsGlobalSettings). It is working, if it would be a sub class of cSigCJComDockingPaneGlobalSettings.

I've dynamically created an instance of cSigCJComDockingPaneGlobalSettings for ComDoPropExchange, which can also be used for ComCreateAsRegistry:

   Function CreatePropExchangeObject Returns Handle
      Handle hoPropExchange hoGlobal
      Variant vPropExchange vGlobal
      If (not(IsComObjectCreated(Self))) ;
         Function_Return 0
      Get Create U_cSigCJComDockingPaneGlobalSettings to hoGlobal //war: ghoSigCjPropEx == U_cSigCjComSuiteControlsGlobalSettings
      Send CreateComObject of hoGlobal
      If (not (IsComObjectCreated(hoGlobal))) Begin
         Error DFERR_PROGRAM "Cannot create the global settings object"
         Function_Return 0
      Get ComCreatePropExchange of hoGlobal to vPropExchange
      If (not (IsNullComObject (vPropExchange))) Begin
        Get Create U_cSigCjComPropExchange to hoPropExchange
        Set pvComObject of hoPropExchange to vPropExchange
      Send Destroy of hoGlobal
      Function_Return hoPropExchange
and have than implemented it in an own sub class of cSigCjDockingManager:

            Get CreatePropExchangeObject to hoExchange // war: ghoSigCjPropEx
            Get ComCreateAsRegistry of hoExchange False ("HKEY_CURRENT_USER\" - sRegKey) to bOk
            If bOk Begin
                Send PX_Delete_Key of (Private_phoLayout(Self)) sRegKey
                Send ComDoPropExchange of (Private_phoLayout(Self)) (pvComObject(hoExchange))

           Send Destroy of hoExchange
notice: this is still a protype - there may be still some errors in it, but it looks like it's working atm.