I think the shure se series with the separate headset cable could be worn straight down. The headset cable connector is 90 degrees. Earphone solutions I think sells them together as a kit.
After a few years of bike commuting with Messenger bags I switched to waterproof backpacks instead. The Messenger bags were killing my shoulder and compressed my sternum when heavily loaded.
I use Orlieb bags and I see more and more...
You could also do what superlux did with some of their phones, and terminate the cable with a jack one inch from the driver, and use an extension cable.