Hi,
The raster element for raster attached in Raster Manager is a bit tricky because even though the raster element is loaded in memory when the DGN is open as most DGN elements the actual raster data the raster element is referencing to is loaded only as required, for the obvious reason that many raster are too big to be loaded rapidly into memory or even fitting into memory. In other words, if your raster is not visible no data from that raster will be loaded until some part of it becomes visible.
That being said it would be useful if you could give us a bit more information as to what you are trying to achieve which requires you to know when the raster finishes loading?
Thanks,
Mathieu