Can iPad users preempt apps so that the landscape orientation is always the default?

No. If an app only works in Portrait mode, you cannot override that. Any iPhone apps downloaded to an iPad will always be in portrait mode and some iPad apps only work in Portrait mode. I had the Mario Run game for iPad and it only works in Portrait mode.

Apps may, or may not, support both Portrait & Landscape modes - this being at the discretion of the App developer. Some Apps may only support one or other screen mode - and will always launch in this mode if the other is not supported.

Where both screen modes are supported by an App, then they should open in the orientation determined by the physical position of the iPad - as determined by the internal sensors of the device.

For example, if the iPad is placed upon an easel (or other stand) in Landscape orientation, then Apps supporting this screen mode should launch in this mode. Similarly, if physically supported in Portrait orientation, then launching an App in Portrait mode would be anticipated.

However, if your device is placed flat on a tabletop, then consider that the internal sensors have no way of knowing which screen-orientation represents “up” from the perspective of the viewer - thus, less predictable results may result!

I hope this is helpful in answering your question.