I have found a possible solution - see here: http://jsfiddle.net/qYdwR/1636/.
The idea was to "publish" the validation errors as a map attribute, validationErrors, through a get function using the map.define plugin. Then, reading the validationErrors attribute from the stache helper, makes the attribute on one hand change its value automatically when something changes in the map attributes, and on the other hand observable. Hence, after the list of counties is loaded from the server, the validation message updates itself.
Observe how, initially, the select list shows "Loading countis,,,", the countyid attribute is set to 3, and the validation message says "countyid is not in the list". And a bit later, after the list of counties is loaded "from the server" (i.e. the fixture in our case), the message "countyid is not in the list" disappears, without any explicit code to do that. It means that the validationErrors is automatically updated after the "counties" attribute gets modified (loaded with data), and that this update triggers an automatic update of the text in the template.
I do not know if there is a more elegant way, so if you have another approach, please post.
I suppose the most elegant implementation would have been if the value returned by the errors() method (exposed by the map.validations plugin) would be itself observable (for example exposed through a "virtual" attr), so that it could be be directly used in the stache plugin without the need to expose it in another attribute of your "business" map.
