I might be missing something, but can’t you just check the check-box in the Geometry palette and have “Auto based on keystoning / horizon” as the Correction setting and then apply the Horizon tool.
This will not allow you to adjust the applied cropping while in the Horizon tool immediately, but if after changing the horizon you then select the Cropping tool without selecting Rest or Apply in the Horizon tool then you are straight into Crop with the adjusted horizon. You can then move the initial auto-crop as desired and once you hit Apply you get the combined effects of the Horizon and Crop changes (and the Crop changes can include changing the aspect ration).
The above applies to all of the Viewpoint perspective tools, not just the Horizon tool.
Personally, I use a preset that has “Auto based on Keystoning” selected and Crop on by default. I find that the auto Crop more often than not produces a decent result and if I want to change it I can just hit Crop as outlined above.