OK, I finally worked things out. The attached file contains three added pieces of code, all clearly marked. The original code has not been changed. If you wish to use it, drop the file (unzipped) in the /home/opt/advancedcaching folder of your N900, replacing the original hildon_plugins.py file.

It will bring you an extra Tool, Dutch Grid RD to WGS. The Dutch Grid or Rijksdriehoekscoordinaten RD works with X ad Y values, in this case in meters. The tool transforms these values into Latitude-Longitude. More info:
http://www.dekoepel.nl/pdf/Transformatieformules.pdf

Maybe Daniel can take it into the standard version of AGTL?

Happy caching in Holland!