If you can tolerate to lose a mm or two from the bottom of the feet, you can perform a Plane Cut.
Open the model, select the Edit button on the toolbar to the left. Select Plane Cut and use the vertical arrow within the image to slide the plane to the cutting location.
Use Control and the middle mouse button (or wheel) to manipulate the “camera” for rotation. Hold shift and middle button to pan the camera. After reaching the bottom, or very near the bottom, position the camera in such a way as to be able to move the vertical arrow for the plane, while observing the cut area. You will see the color change that represents the new plane/new bottom of the model.
When you click Accept, all of the model in the direction of the arrow will remain, while the plane cut becomes a closed surface.
Export the STL when completed.
Zoom uses ctrl-middle button- mouse movement.
Following your original suggestion of rotating the model, it is necessary (or rather, preferable) to create a pivot point near the feet. Import the model, select Meshmix from the left toolbar, drag a sphere into the work area, between the model’s feet. Use the tiny white box in the center of the sphere to reduce the size of sphere. The smaller, the better.
You should see the object browser appear when you’ve added the sphere. Hold Shift, click on the model to select both items. Select Edit, Transform to get the manipulator, this time centered on the sphere.
Holding the arcs visible in the manipulator will give you rotation about the center of the sphere, and include rotation of the model. After grabbing an arc, move your mouse as far away from center as you can for finer control.
I think it’s going to be tricky to find a flat-foot in the latter method, while chopping off some of the feet will be easier, but you may have more patience and a better eye than I.