It is possible, I just named my presets.xml the same (without the custom- part). You could alternatively add a new preset or a few and extend the user creation webscripts to use that preset instead of the default one.
But when you say you want to change that dashlet - in what way? Because in presets.xml you only change it for new users. Existing ones will stay with their old dashlet configuration. You should practically browse each users' avm store and modify those XML config files.
