How would you want to mount it to the pannier rack? I don't think I've ever seen anything that mounts to a rack and works as a messenger bag (never mind looking non-bikey off the bike). Most rack packs (Carradice Super C, Arkel Tailrider, &c.) have a shoulder strap, but can't really be worn as messenger bags. A small messenger bag (e.g. BigxTop mini messenger) could be strapped to a rack, but it might be a bit of a bodge involving bungees. If I had to have a bag off the bike all day I'd probably get something suitable for that (and with just lights and valuables it could just be a musette?), and then get something like a basket and cargo net to carry it securely under way.