The customer asked for the feature which allows him to decide which siteaccess should be visible in the particular backoffice.

The customer has few backoffices, each per siteaccess group.
The first for SiteA site has two languages, the second for SiteB also, for instance:

ezpublish:
	siteaccess:
		list: [sitea_fre, sitea_eng, siteb_fre, siteb_eng, backoffice_sitea, backoffice_siteb]
		groups:
			sitea_group: [backoffice_sitea, sitea_fre, sitea_eng]
			ger_group: [backoffice_siteb, siteb_fre, siteb_eng]

So, he wants to see a restricted list of siteaccesses in Landing Page editor accordingly.
In the first backoffice (for SiteA), he wants to see only sitea_fre and sitea_eng siteaccesses. In the second backoffice (for SiteB), he wants to see only siteb_fre and siteb_eng siteaccesses.