What will the addition of 1 tsp. barley malt extract do to the dough for a loaf of sourdough? Any pros or cons?
If you're doing a long, slow ferment, then the malt syrup is just added food for yeast. One teaspoon won't do much of anything, but too much and fermentation grinds to a halt. I don't know why. (Maybe the yeasties get drunk on malt syrup happiness and just go to heaven or sleep or something). I've ruined several loaves with too much malt syrup extract or molasses or other syrups.
I seldom use it for anything beyond bagels. (I add the extract to the boiling water pre-bake).
If what you're asking is "Will it affect the flavour?", not really. If you want that flavour, buy some malt from a beer store, grind it (in your coffee grinder, etc.), and add to the dough, just not too much, say 1 tablespoon ground (and move up or down per your preference)
I added malt and baked a loaf. Indeed it did increase fermentation time by about an hour. Didn't change flavor much but it really darkened the bread. I see no advantage it using it.
If you want malty flavour, use ground malt instead of malt extract.
You can buy a number of malt varieties from a local wine/beer store (not a liquor store, per se, but a store where they sell all of the things you need to make your own beer or wine; I guess a "homebrew" store might be what they're called).
They'll likely have a grinder there, but I prefer to take the whole malt home and grind it my coffee grinder just before adding it to the dough.
You'll need quite a bit to affect the flavour; I use 1/4 cup per loaf.
Malt will change the colour of the crust, but not consistently, in my experience. I use a Caramel 80 malt to make reddish-coloured (rust-coloured) rolls. Sometimes it works, but sometimes it doesn't.
Web page on different types of malts used in beer (and can be used in bread): http://www.howtobrew.com/section2/chapter12-1.html
If you want a malty flavor you can also do a mash. I generally use about a tablespoon of malted wheat flour added to the portion of the flour you use for the mash. You then add water at about 165 degrees to bring the temp of the flour/water mixtuer up to around 150-155 degres then pop it in a 150-155 degree oven for about 3 hours. this technuique if often used in German breads such as pumpernickel or Volkornbrot.
I sometimes use malt extract for veinna bread, bagels or rarely in pretzels but not much.