I do not think MasterCam will allow this. I have mistakenly run into this problem before when accidentally constructing a solid tool body for removal inside another solid and then tried multiple ways to select it for removal. Once I moved the solid tool body for removal 0.001" outside the other solid, I was able to select it then.
Besides, why would you want to do this? If the created feature would be entirely enclosed by the main solid, you would not be able to machine it anyway.