You can call Sketchup::Model.place_component if you are actually placing a component instance. (If you do this from within a custom tool, the native Move Tool will be activated for the placing, and your tool will no longer be active.)
Here is a code example that “mimics” the Move Tool using Sketchup::Model.place_component :
I also posted another example of a Drag Tool that uses a transformation to move an instance instead. (This can be used within a custom tool.)
The whole of this topic thread has interesting information (for context.)