The second half of your statement is where I see the problem. Set aside all of the licensing, regulatory, and investment requirements for sake of argument.
"A half barrel of 'house beer' would sell very fast even if it was mediocre just because of the customer volume."
So, if you're looking to capitalize on customer volume and sell lot of beer, you've got to make lots of beer. In order to even make that worthwhile, you're probably looking at a 7-10bbl system with fermenters, glycol, serving tanks and grain storage. That's a very significant investment in equipment, space and additional labor just to make a few extra bucks per keg.
Around where I live a number of bars have "House beers" but no brewery. What they do is usually contract with a local brewery to buy some portion of generic amber or pale lager and just claim it's their own beer. In some cases, local chains have a custom beer made for them - but again, this is a contracted brewing solution, they're not making any beer themselves.
Then of course, there's a bar down the street has a "house beer" that is just a keg of PBR.
If the customers would willingly guzzle crap beer, then why bother with the effort of making anything custom for them? Just get the cheapest acceptable keg, charge an extra dollar a glass and tell them it's the house beer.
