There's a guy here on bikeforums who has a beautiful website including photos of his fleet of Xtracycles - most of them are coaster brake models, and he apparently has no troubles with them. I don't know exactly how much he puts them through, as I would certainly find a coaster brake insufficient carrying large amounts of cargo - particularly downhill.
The website is
http://bikerubbish.com
You could always get a new front fork with some brake tabs on it, that wouldn't be too bad - finding one that looked good on the Straight 8 might be harder though
Changing a flat on an Xtracycle isn't really any more difficult than it used to be, from my experience. There's no special mechanism, the wheel is held in place by a normal vertical dropout.