I'm not sure you can display blocks here as they are quite specific to the layout datatype that's part of ezflow.
What you can do is add the following line to your user/login.tpl template
{ezpagedata_set( 'extra_menu', 'user_login' )}
This tells the pagelayout.tpl to show the right menu (adds the extrainfo class to the page div) generates the extrainfo-position & extrainfo divs and loads parts/user_login.tpl into the extra_info div
If the value set to extramenu is an array then a template for each item in the array will be included.
Thanks for your reply, I do not know this use of extra_menu.
Another solution I found is to create somewhere in the content tree an object of a class using the layout datatype, then put the node_id of this node into a ini file.
Finally I fetch this node, I loop other blocks of the layout attribute, and I display Those that interest me using node_view_gui... That's not very clean but there is no many solutions I think !