That's awkward because I like to use TrailGuru as I walk to go pick up my youngest from school in the afternoon. After a bit of hunting, I've located a way to detach the battery from the phone to create a bending solution that does fit.
I recently picked up a Male-to-Female dock extender from DealExtreme for under four bucks (with shipping included). The cable lets me "fold" the battery into place for afternoon walks or stick it into my pocket while using the iPhone on a tether.
As far as stroller and backpack use goes, it seems to be working quite well. As for tethering, it's a fairly thick cable, and brings with it all the standard tethered irritations. Frankly, I'm not a big fan of tethered use but when you need that extra battery charge, it's a slight improvement over having the battery pack stuck directly into the bottom of the unit.
DealExtreme ships from HongKong, so it generally takes a week or so for packages to arrive. I've bought lots of things from them and they've been reliable shippers. When purchasing from Chinese suppliers, do keep in mind that the individual item fail rate is going to be higher than you might expect: not a constant issue, but something you should factor into your purchases, and something I've certainly experienced.