I would also guess the altitude is what wrecked your muffins... I try and use the same "concept", but don't have that cookbook myself (though borrowed it from a friend to get ideas). i like to add shredded carrot and apple to almost any muffin and it still tastes good. have you tried the mountain muffin lady's cookbook? they are all high altitude adjusted and pretty easy recipes to follow. my daughter likes the banana blueberry muffins a lot.
for my meatloaf, I shredd carrots, zucchini & onions.. then kind of dry saute them with a little bit of olive oil in a non-stick pan. this cooks them lightly, but also removes some of the moisture. here is my recipe paraphrased (its a big meatloaf)... I turn the leftovers in to sandwiches, spaghetti or even taco meat:
1 pound ground beef (90% or 95% fat free)
1 pound ground pork
1 egg
1 cup breadcrumbs
couple dashes of worsteshire sauce (sp?)
couple dashes of hot sauce (tabasco or other)
tsp of dijon mustard
1 tsp salt
1 tsp pepper
herbs of your choice.. I usually throw in a little thyme or italian seasoning
1 tbsp fresh chopped parsley (dry is o.k. too)
mix well with cooked veg (i throw in my large mixer). if you mix by hand, use your hands and really work everything together... it helps bind everything as well as work the pork fat around (since you are using such lean hamburger).
bake on cookie sheet not a loaf pan (make a kind of flattened "log"), this helps it not retain so much moisture too.
also, I find that most of the purees are best hidden in stronger flavored items, like spaghetti sauce or soups. good luck!