I don't know of any standards. I agree that it's frustrating. The ubiquity of cheap 16:9 panels design for television makes it understandable that these digital frames aren't available in more photography-friendly aspect ratios.

I think one reason that there's no standard is that it really depends on your content and what you're presenting. If the images are primarily art, the scrolling effect is probably detrimental — but if they're documentary, then it may be ideal (there's a reason it's called the Ken Burns effect, after all).

Cropping to the frame's ratio is likewise very content- and subject-dependent. Even where it can work without losing parts of the subject, a different framing can make a big difference (see What should I consider for cropping aspect ratios? for some examples).

And, when padding (or "letterboxing") images, the question of black, white, another solid color, or a pattern — well, same story here. It depends on the image. Overall, this is generally the least destructive, so it's my preference for a default — but if you're trying to fit a 2:3 portrait image into a 16:9 frame that doesn't easily rotate, that can be awkward.

Even the stretching option might work in some cases (although... as you say, usually not).

Sorry I don't have a better answer, but I don't think there is one. The best practice, I think, is to consider and do the best thing for each individual image, or, if all presented together, for consistency as the best for the set of images.