To be honest, it's irrelevant whether the belt is between you and baby in a sling, or around you both. In an accident, you're likely to jackknife and crush your baby regardless of seatbelt position. Believe me, I've experienced the jackknife effect, having been in a taxi that rammed into a line of traffic in front of it. Thankfully, this was years ago, pre-children. I was in a neck collar for 3 months, had physio on my neck for 6 months, had a full body bruise in the exact shape of a seatbelt - I wasn't even badly injured, but if I'd had a child on my front, I'd have killed it for sure with my G-forced body weight.
It is infinitely safer to have your young baby in a capsule seat, and an older child on some kind of booster (and now that the supremely portable BubbleBum is available, there's really no excuse for saying it's not practical to carry a booster around). We've used capsules, junior carseats, boosters, sit n strolls - anything to protect our children. And yes, it's inconvenient. But if there's an accident, at least they stand a chance.
