There is no right or wrong answer here. It's all about negotiation power.
If this band is powerful enough to demand that only 1 brand of beer be sold at a bar (I doubt it) then the bar will agree to it.
If the band says "You've got to sell this one beer or we won't play" and the bar says "OK, then you don't play" then you know the score.
A much more reasonable tactic would be to get the bar to agree to FEATURE the beer that night/weekend. Do special pricing - put out branded coasters and the table top adverts mentioned above - give out some other "premiums", etc. Demanding or even suggesting that a bar only sell one beer would likely seen as insulting and very assumptive.
The band and the beer-maker can really make out with a strong relationship, but their going to have work WITH the club-owners on reasonable promotion in order for it to be a WIN-WIN-WIN situation.