Regarding Moti's comment above about getting the build results out of the build definition:  It is possible to get the build results from the build definition.  I'm including a method below that queries for a specific build using a label and build definition, but you could pull out the code related to the label:

    public IBuildResult queryForBuildUsingLabelAndBuildDefinition(String label, String buildDefinitionId, ITeamRepository teamRepository) throws TeamRepositoryException {        
        IBuildResultQueryModel buildResultQueryModel = IBuildResultQueryModel.ROOT;
        IItemQuery query = IItemQuery.FACTORY.newInstance(buildResultQueryModel);
        query.filter( query.and(buildResultQueryModel.label()._like(query.newStringArg()), buildResultQueryModel.buildDefinition()._eq(query.newItemHandleArg())));
        query.orderByDsc(buildResultQueryModel.buildStartTime());
        IProgressMonitor monitor = new NullProgressMonitor();
        ITeamBuildClient buildClient = (ITeamBuildClient) teamRepository.getClientLibrary(ITeamBuildClient.class);
        IBuildDefinition buildDefinition = buildClient.getBuildDefinition(buildDefinitionId, monitor);
        Object[] parameters = new Object[] { label, buildDefinition };    
        IItemQueryPage queryPage = buildClient.queryItems(query, parameters, IQueryService.ITEM_QUERY_MAX_PAGE_SIZE, monitor);
        if (queryPage.getSize() == 1) {
            return (IBuildResult) teamRepository.itemManager().fetchCompleteItems(queryPage.getItemHandles(), IItemManager.DEFAULT, monitor).get(0);
        }
        else {
            return null;
        }
    }