How do I add a new configuration item to registrymodifications.xcu

Hi,

I am working on an enhancement for Calc, tdf90180.

Therefore I need to add a configuration item to registrymodifications.xcu.
I have created a class to obtain the configuration item, using a derivative of 
::com::sun::star::utl::ConfigItem and copying/adapting existing code to retrieve configuration 
items.

When the new configuration item is not yet in registrymodifications.xcu and upon opening a relevant 
Calc document I get an exception:
"warn::unotools::config:7006:7006:unotools/source/config/configitem.cxx:426: ignoring 
XHierarchicalNamesAccess to /org.openoffice.Office.Calc/Content/ErrorRectangle Exception: 
N3com3sun4star3uno9ExceptionE msg: ErrorRectangle"
When I manually add the configuration item to registrymodifications.xcu and open a relevant Calc 
document I get the same exception.
After running soffice (or scalc) my manual entry to registrymodifications.xcu is no longer there.

I have grepped similar -existing- configuration items in the code, and could not find anything that 
writes the item to registrymodifications.xcu.

Is there a simple way to fix this?
The only alternative I see now is to submit an incomplete patch to gerrit for review; I would like 
to avoid that.

Winfried