I totally understand where you're coming from but unfortunately it's just not in the cards this time around. If it were something I could pull off in a week then I'd be up to considering it as a possibility. As it stands, due to the inherent limitations of PixelCNC's core systems any given XY point on the canvas can have only one corresponding Z coordinate. As as a result there is no way to cleanly or easily add any kind of functionality for designing or calculating toolpaths for any kind of overhangs. This applies to the simulation system and cutter geometry as well. Everything is built around a simple 2D representation where each point can have a single Z coordinate tied to it.
The best I can do is add a new entry-cut mode to the profiling operation which instead approaches cuts from the side, rather than from above, but you'll still be defining your router bits as one of the existing tool types and simulating it without being able to see what the final result will actually look like in the simulation, but the resulting toolpath should be perfectly usable with a router bit (assuming that your toolpath parameters are correct and it doesn't try to approach the toolpath wrong or move too far into the material).
As for actually giving the user the ability to model cutters or features on their project's canvas that cannot be represented using a 2D heightfield function it's just not going to happen without spending months of time gutting tons of code and re-writing it to work with a new canvas representation - including all the toolpath generation algorithms. That's time I just don't have, unless you'd be willing to start paying me a full time salary to work on it. I've already set my sights on getting PixelCNC to beta this summer for over a year now (and it's already going to be really tight as-is) so that I can finally switch to PR/marketing mode and begin generating an income for my wife and kids.
In the meantime, I still have at least half a dozen new toolpath types I plan to incorporate which will allow for some cool new stuff, and a bunch of canvas layer editing functions, along with built-in tutorial/walkthrough modes and a system for orienting new users with the interface.