Probably can be done as a service:

SERVICE wikibase:geolookup {
  bd:serviceParam wikibase:location ?location .
  bd:serviceParam wikibase:data 'World Countries Outline.map' . 
  bd:serviceParam wikibase:globe wd:Q2 .  
  ?wd wikibase:property 'wikidataId' .    
  ?iso wikibase:property 'isoCode' .      
}

It can look up the map on the call creation stage (be careful with timeout though!) and then on call stage just lookup the bindings against the existing map. Check out LabelService.java as an example of the service that does something similar.

Also see https://wiki.blazegraph.com/wiki/index.php/QueryHints (esp. runFirst and runLast) for the way to control when service runs (you probably want runLast).

you probably want runLast

Won’t that just result in T175840: Using label service twice in one query results in obscure error message?

Label service is also very slow, like 2 times slower than to just query labels a normal way, considering that map data processing is probably more complex than just fetching a label I am afraid that it won't work for any real queries with current timeout…

In T179991#3754444, @Base wrote:

Label service is also very slow, like 2 times slower than to just query labels a normal way, considering that map data processing is probably more complex than just fetching a label I am afraid that it won't work for any real queries with current timeout…

Label service is a disk IO operation. For every object, it must load some/all available labels, and pick the best. This service is fully in-memory and CPU bound -- you load the map once, cache it, and afterwards each point lookup is just an rtree search in a memory data structure.