Bikepacking - a viable alternative to racks & panniers

You'll regret asking me to elaborate, I tend to be a little too loquacious on bike forums ;)

I tried a Swift Zeitgeist on a Bagman Expedition QR support, on a 200k and a 300k.

Regarding the bag:

• It fit everything I needed, however the bungee cord opening makes it difficult to access the contents. Having a single huge space makes it a pain in the ass to find anything. Basically, I'd prefer a bag with a U-shaped zipper (and maybe a storm flap, for style) that allows it to open completely, and movable, internal dividers (like a photography bag).
  
• Because the weight is placed fairly high and the bag is wider than my handlebars, it makes it difficult to rest the bike somewhere and look into the contents of the bag. It's a detail, but an important one during audaxes for me.
  
  

All of this could be fixed by having a custom bag made. However, my main criticisms are about the Carradice Bagman:

• The clamp is huge and limits the range of adjustment of the saddle. I don't want to start relying heavily on a support that I might have to get rid off in the future because of bike fit. I'm seeing a fitter in September: if he brings my saddle forward by more than 3 mm, I just won't be able to fit a Bagman anymore.
  
• It's made my saddle move backwards several times, on a properly torqued Thomson Elite. I'm not sure whether it's a slow process due to the vibrations, or something that happens when I hit a big bump, but I'm not willing to buy the additional struts to find out, due to my other concerns listed above.
  
  

I know alternatives exist, such as the SQR from Carradice or the Unirack from Manivelle, but given that my thighs are touching a seatpack, I'm fairly certain they'll touch these supports as well.

@Tijmen : Thanks! I had considered it but decided against it because it still touches the seatpost, so I don't think it'll solve the issue. My thighs are avoiding my seatpost by less than 5 mm on each side, basically.

@Samuli: Oh yeah, the Gondola might be perfect. Unfortunately I can't find a shop in EU that sells it (I'd want to try it and return it if it doesn't work for me) :/

I might actually go with a rear rack and slap Ortlieb accessory packs on each side... It seems that's what every ultra racer is doing with the Tailfin rack nowadays anyway.