I worry that this anti-Walmart meme could lead to an odd left-wing resistance to GBI/more lavish welfare state, since the policy would be branded as a subsidy to Walmart.

I don't understand why you are worried that the anti-wal-mart meme would cause anyone, let alone a leftist, to oppose GBI ... a GBI reduces the incentive to work for low wages. If it's true that welfare is benefiting Wal-Mart and other low-paying companies, then the reason is that in the current system you lose your benefits if you don't get a job. This is just the natural consequence of our method of imbedding an incentive to work within the welfare system.

I suppose someone who didn't like GBI and thought work-dependent welfare was an acceptable alternative might be driven to oppose welfare in addition to GBI upon learning about this side effect.