Just a couple points to lob in here:

Someone mentioned that the carabiners sold at Target/etc aren't as strong as proper climbing ones sold at REI. Truth. But realise the belt loop on a pair of jeans was never designed to take a serious load(when you clip something to your belt,it spreads the load across a couple loops). Also,no offense but I don't see the point of doing this. If you're wearing your bag properly,it's high on your back and a carabiner prolly won't reach a belt loop. I've never had an issue with my T2 bags sliding about when the bag was up on my back and the cross strap was used.

Misha: you do realise the Eula and XS T2's are basically purses,right? I mean they're pretty small and don't have much capacity. You certainly won't be hauling any goceries with them. If that's what you want,fine,just giving you a head's up. I have a small that I've used as a walking around bag,it's ok but also doesn't haul much. A medium isn't so big as to be unwieldy,but can carry a fair amount of stuff.

My Courierwear bag didn't have a cross strap when I got it. I added a T2 strap by putting a D ring on one of the flap straps. I unthreaded the buckle off the strap(had to pop the stitches out of the tip),slid the D ring on,then replaced the buckle. Then I just clipped the T2 strap to it. QED

Also,FYI for anyone in the market for a T2;if you're not super picky about color eBay and various outlet sites carry them for as low as half off MSRP. Steepandcheep has them sometimes too.