Hi Scott,
There's no out-of-the-box way to do it unfortunately, but it's easy to do by changing the BingMapsImageryProvider. You just need to replace 'http' with 'https'. Here's a hacky but working approach:
Change BingMapsImageryProvider.js, around line 128 to look like this (new line in bold):
var metadataUrl = this._url + '/REST/v1/Imagery/Metadata/' + this._mapStyle.imagerySetName + '?key=' + this._key;
metadataUrl = metadataUrl.replace('http:', 'https:');
var that = this;
And around line 412:
var url = buildImageUrl(this, x, y, level);
url = url.replace('http:', 'https:');
return ImageryProvider.loadImage(this, url);
I had hoped that just passing in a new URL with 'https' to the BingMapsImageryProvider would do the trick. But unfortunately the result of the Bing Maps metadata service still includes http URLs even if accessed via https.
Kevin