I know what you want to do is possible and seem to remember a thread about how to do it was posted within the last year. I've no doubt somebody else here will be able to help on this. But there are alternative options which you may consider just as suitable.
I realise that some users want to set things up in a particular way for their own reasons and I'm not trying to persuade you otherwise. However I wondered if have you thought about using UnleashX as your main dash?
The shortcut button boot options UnleashX provides can launch XBMC with a simple button press on the controller or remote when you turn the Xbox on. To my mind this is more convenient than having to press the eject/close button twice unless there is actually a disc you want to load of course.
I have the MS Dash, XBMC, an older UnleashX version, Avalaunch, a SID dash and Evox boot shortcuts set up to various buttons but TBH I rarely use them. It is hardly any more trouble or time launching them as applications from UnleashX or any other main dash.