The sourdough recipe I use calls for 300ml of water to 500g of bread flour (along with the starter). Instead of this water, could I use beer - or a combination of beer and water? The reason I thought this would be possible is because the beer would add taste and air to the sourdough. Along the same lines of adding more air to the sourdough, would sparkling water be a good idea, if beer is not?
You can easily replace the liquid in most bread recipes with beer. This can have a very pronounced effect on your final dough as there is a lot more chemical and biological fun happening in beer than there is in water. In my experience, the dough with beer will usually rise faster than a similar dough with water. Generally, the flavor difference won't be that pronounced (usually a much more "bready" flavor, unless you use a beer that otherwise has a strong flavor, such as an IPA).
The acidity of the beer won't actually have that much of an effect on your final dough as the ph will be a weighted average of all of your ingredients.
You're going to lose most of the carbonation when you pour in the sparkling liquid. You may trap some, but I'm guessing not enough to make a difference.
As for using beer for flavor, it sounds like an interesting experiment. Beer is much more acidic than water, but thankfully yeast likes an acidic environment. While I wouldn't replace all of the water with beer, judicious substitution of some water with beer shouldn't upset the pH of the mix beyond where the yeast can survive and multiply.
Bear in mind that pH will drop as fermentation occurs since fermentation itself produces carbonic acid.
I have been using a starter I started with a Belgium blonde for some time now. I feed it every twelve hours because it stays out all the time. It has a great funky sour taste. I use it in everything. Never really worried about pH. It bubbles away and raises nice. I do sometimes add yeast though.