Hierarchical Clustering customized Linkage function
Fork sklearn and implement it yourself! The linkage function is referenced in cluster/hierarchical.py as


  join_func = linkage_choices[linkage] 


and 


  coord_col = join_func(A[i], A[j], used_node, n_i, n_j)


If you have time, polish your code and submit a pull request when you're done.