Ok - thanks TIG - i tried again over the weekend.
Unfortunately i can't get it working the way i want. I think the available observers and methods are just not providing the necessary functionality.
I can intercept the default MoveTool for a component instance contained in the model using a ToolsObserver and let my tool run instead - fine.
I can spot when an instance was placed using a ModelObserver with onPlaceComponent and let my tool run - fine
But i can't intercept the placement of a component before an instance was actually added to the model.
The ToolsObserver fires when i click on a component in the components browser but doesn't provide info on the chosen definition.
On that first click in the browser the definition is added to the model's definitionlist (only once as i stated before) and a DefinitionsObserver can spot this (once).
The component now sticks to the mouse pointer and i can hover over the model. There's no change to the model's entities collection yet.
The instance is placed when i click (a second time) somewhere in the model.
Now the component instance is added to the model entities collection and at this point an Entities-Observer or Model-Observer (onPlaceComponent) can spot it.
But this is already too late for visual feedback while placing.
Nevertheless... keeping on sketch(up)ing