What you can do in this case, for instance to have black street labels over the street basemap but white street labels over the aerial basemap, is to put the street labels map services inside the <basemaps> tag and give them the same layer label name as the basemap you want for the combination. By this I mean give both the streets basemap service and the black street labels service the same layer label "Streets" and give both the aerial service and white street labels service the layer label "Aerial." When the user selects the Streets basemap they see black labels, and when they select the Aerial basemap they see white labels. The tradeoff with this configuration is that the user cannot turn off the labels as when they are used as operational layers.
Hope this helps.
Marc