Could I use my phone to scan or image myself? Could I use my phone or tablet to create an image rendering of my body that I could use to quantify body metrics like waist size, inseam, etc?
No. Even humans, who are much better at vision and understanding pictures than computers are, can't determine accurate dimensions just by looking at a photo.

The closest you can get is apps like in Is there any app that measures the distance using the camera?, but they can only answer simple questions using trigonometry: if you know how far an object is from the camera, it can measure how big it is, or if you know how big it is, it can work out how far away from the camera. Even then, they can only give very approximate answers. An app like that wouldn't be able to tell the difference between a 30" or 34" waist, for instance.

The only way to use your phone to "create an image rendering" with this data would be to use a drawing or 3d modelling app to make the picture and then add labels for measurements you took with a tape measure.