*cough*

-Messenger bags that swing around are not true mess bags. They are shoulder bags that lack the stability strap.
-Seat post rack will not support too much weight.
-"Backpacks sit up too high/too small" have never (probably) seen a Banjo Brothers Backpack. They have some looong straps compared to other packs that I have owned over the years.

@OP- depending on your budget, you might want to check out Carradice saddlebags that attach to your saddle. Also, if you aren't too leary of buying from e-tailers, look into what the Europeans have to offer. Germany's Rixen and Kaul has some nifty products.