There's a lot of things like this, that you have to cheese to get around.

1) If you mount a block on a wheel, then propellers to that block, then remove the block and replace it with a wheel (using the part placement bug), the propellers will be mounted to the inside of the wheel.

2) You can't place a bomb on a grabber, but you can place a block, put the bomb on THAT, then remove the block and place the grabber there.

3) You can't place armor on a block that has a strut connection in the way, but you can place the armor, then peer UNDER the armor and place the strut

4) You can place anything ANYWHERE by turning on all the graphics settings, (to tank your FPS) and then quickly moving your mouse over the place you want to place something and clicking (before it turns red)