Others have requested this as well, so I will have to consider adding the option to pivot the steerer tube inside the head tube for a future update. In the meantime, I've come up with the following calculator that you can use in conjunction with the Lock frame tool in BikeCAD to see the effect of adding an Angleset headset to your existing frame. Before, I describe this calculator, I should point out that BikeCAD is already well equipped to show how using an Angleset headset would affect trail. The trail dimension is solely a function of head angle, fork offset (rake) and front wheel diameter. If, for example, you use an Angleset headset to make your head angle 1° slacker than your current setup, you can safely take the BikeCAD model of your current bike and subtract 1° from the head angle and see what the new trail dimension will be. Of course, as you've pointed out, doing this will result in a model that is no longer an accurate representation of the rest of your bike. However, that is irrelevant to the calculation of trail as none of those other frame dimensions have any impact on trail. Once you've taken note of the new trail value, you can revert back to your old model.

The calculator below was created in recognition of the fact that using an Angleset headset will cause your frame to rotate about the rear axle having a very subtle effect on seat angle, stack and reach and other frame dimensions. To use this calculator, model your current bike in BikeCAD and then click the Lock frame icon. Use the calculator below to determine a fudged fork length and a fudged fork rake. Enter these fudged lengths into BikeCAD. and observe how your frame has rotated about the rear axle affecting dimensions such as seat angle, stack and reach and the actual angle of your head tube. What the fudged fork dimensions do is locate the front axle where it would be if your current fork were pivoted about the fork crown by the angle specified in the Angleset angle field. It should be noted that this approach should give an accurate representation of how your frame geometry will change due to the rotation about the rear axle, it is not going to give you an accurate representation of trail. It also neglects to take into account how your stem angle will be affected as it will since it's attached to your steerer tube.

Don't be alarmed when you see that the head angle dimension shown with the new fudged fork dimension has actually changed slightly in what seems like the wrong direction. Remember, this is the angle of your head tube, not the new head angle which is the angle of your steerer tube which is passing through your head tube at an angle.

Let me know if you have any questions or comments.