The wait is over. I have cried tears of joy.
This past Monday, Automattic announced a new feature: email forwarding for your custom domain on WordPress.com. It is no longer necessary to manage a separate email solution for your domains on WordPress.com; forwarding can be set up from Store → Domains within your dashboard. If you already have another solution set up, just remove its DNS records to enable the native WordPress.com feature.
This removes the last frustrating roadblock to having an all-WordPress.com web presence. So go start that blog, or build that new site for your business, knowing that email for your domain will be as simple as everything else on WordPress.com. Hurray!