Gotcha OP. Basically, there is a clash between artists who want a really wide canvas for their art and audience who want a less wide canvas and is willing to chop the edges of the artwork off to get it.
OP, to do what you want to do you can do a little math and use Handbrake cropping. If you have an 816 line movie and you want 16:9, you'll crop it to 1450 to fill the screen (no black bars). There's online tools you can use for other amounts of lines. For example:
http://size43.com/jqueryVideoTool.html If you can live with some bars, just pick the level of OAR compromise you'll accept and use that info for your crop values.
It sounds like it's a thickness of black bars issue, so you'll probably choose something between 1920 and 1450 wide or 1080 and 816 lines tall.
Handbrake, (choose profile like

TV), "picture settings" & punch in your crop values, "start". If you think you'll watch the movie on other screens in the future, you might want to keep a master copy in OAR. On an iPad, really wide screen movies can be a relative pain to look at it. However, on a big screen TV, the black bars can almost disappear in the right room (and you get to see the
whole picture). Again, you need to run only one old western where the gunfighters are at the far left & far right of the screen (areas you've cropped off) to really appreciate sticking with OAR. War movies, some Sci-Fi, etc can also put important stuff to see at the extreme left & right.
OP, you also reference wanting to upscale so that the end file is 1920 x 1080. Upscaling is inventing pixels that aren't there. You're asking a computer to guess the color of the invented pixels. So again, you're going even further from what was intended. If you want to do this anyway, it's a crop job and then an upscaling job. For instance, you might crop the one you mentioned down to 1450 x 816 to maximize your lines, then upscale that to 1920 x 1080. You'll be asking the system to invent the pixels between 1450 and 1920 and you'll be asking it to invent the lines between 816 and 1080.
If this is about filling or nearly filling an iPad screen, you can just render the cropped file at 1450 x 816 and then use the "red button" to zoom it up to nearly fill the screen. In other words, you can just let the iPad dynamically upscale it for its screen. This would keep the file size smaller and yield pretty much the same result for your eyes on playback.