It is possible to attach sustainment pouches to the side of an ALICE ruck, - this really limits what goes into the outside pockets on each side. The pouches will also flop around a bit. (Shrug) YMMV. Just thought it was interesting to see.
I'm setting myself up a Hellcat right now, mostly because I had a large ALICE pack + frame with a broken belt, sitting around gathering dust, and I figured I might as well turn it into a project rather than toss perfectly good components. Worse case I figure I'll set it up and give it away at a ZS20 charity event or something. Anyway, I bought a MOLLE belt on Ebay for $6 + $6 shipping, and even though it hasn't arrived yet, I happened by my local army-navy surplus store, and they had Sustainment pouches for $5 each. That beats anything I could find online and no shipping, so I bought a pair. They DO attach to the sides of the Large ALICE, but I wonder if we're attaching them differently.
This website has photos of a Large ALICE with the MOLLE sustainment pouches attached; this is how I attached mine: http://shop.cjlenterprize.com/Molle-Sus ... 2POUCH.htm
The sustainment pouches are attached to the far outboard web straps, and don't seem to interfere with the outer pockets at all. However, the MOLLE straps on the pouches are farther apart than the webbing on the pack, so when I install them like that it forces the fabric of the pouch to bunch up a bit between the straps. The top lids look a bit rumpled, and I'm sure it impairs their internal volume a little...but they're still a huge addition to the volume of the pack. Filled with towels and the pack filled with pillows, it looks wide, puffy, and not too floppy, but I'm sure with enough weight in 'em they'd sag like...well, you know, like two large, floppy sacks full of weight might sag.
I am 100% certain that sewing even little bits of molle webbing on either side of the ALICE webbing straps would allow them to sit properly, but that's an added layer of investment in this project I might or might not get around to at some point.