Oh, I didn't eve noticed that that message got sent. It was obviously not finished but you answered my question so thanks a lot!
So here is was it looks like:
this.MbApiInterface.Library_SetFileTag(fileUrl, (Plugin.MetaDataType)Plugin.FilePropertyType.PlayCount, value.ToString());
this.MbApiInterface.Library_CommitTagsToFile(fileUrl);
Thanks again.