Can a balloon and wool be seen as two terminals of a battery?

So is is correct to view the two terminals of a battery as the balloon and the wool, just in the battery there's a chemical reaction making sure that the charges don't balance, rather kept imbalanced?

Parts of that model are fine, parts are not. In the cell, the chemical reaction drives the potential. In the balloon and wool, it's the interaction (rubbing) that drives the potential. The materials by themselves don't do it. If you could have something that rubs them together whenever the charges dissipate, they would be similar.

can we say that there is voltage across the balloon and the wool? How is it measured?

After rubbing, yes there is. When you bring your hand near one and you hear the popping of sparks from the hairs, each pop is the breakdown of the air as the potential difference ionizes some molecules. That only happen when the potential difference over a particular distance exceeds some value.

The difficulty is that for conductors, the high speed of the electron flow means that they have basically a single electrostatic potential. For insulators like latex and wool, different portions have different charges and therefore different potentials. The object as a whole doesn't have one single potential. (It's like asking about the altitude of Colorado)

And measuring it might be difficult. For a bulk conductor, we can get a voltmeter that will harvest a little bit of the charge to measure it. Often that can be done without wildly affecting the object being measured.

But when you try this on a balloon or wool, you're only measuring a teeny, tiny dot of the material where your probe touches. And when the voltmeter makes contact, it probably discharges that tiny spot without properly displaying the value.