In a variable annuity such as the MetLife Series VA, if inflation rises, is it likely or even possible that the "5%" income account could increase?

Very good question.

In most cases, there is no guarantee.  

I suppose people could argue that there is some correlation between equity values and inflation.  If so, then increases in inflation would be accompanied by increases in equity prices.  

Equity price increases would presumably increase the value of variable annuity subaccounts.  This, in turn, would increase the value of the VA's account value and presumably its guaranteed income base.

All of the above rests on pretty shaky ground.

Real, contractually guaranteed inflation adustments to the income base are at best rare.  They may now also be non-existent.

Penn Mutual used to have a VA rider referred to as the Penn Mutual Purchasing Power Protector Benefit.  Moshe Milevsky had a great piece on this feature in Advisor One: http://www.advisorone.com/2010/05/01/annuity-analytics-penn-mutuals-ppp-...

I think this Penn Mutual rider did exactly what you are asking.

Problem is that I think Penn Mutual discontinued the rider.