Dynamic data on overlay layer in openlayers

You can do it with features on a vector layer. Provide a datasource that has features on the same coordinates than your features on the raster layer. Then you can access the attributes of these features. For example, population:

myvectorlayer.features[myfeatureindex].attributes.population = 10000;

(or use one of the myvectorlayer.getFeature* functions)

From now on, you can add a label for the features with Openlayers.Style :

var myStyle = new OpenLayers.Style({
  'label' : '${population}',
  'fontSize' : '8px',
  'fontColor' : 'white',
  'labelOutlineColor' : "#000000", 
  'labelOutlineWidth' : 2});

Where ${population} is the attributes name on the vector layers features.