Is it possible to emulate effect of for_each from plugin SDK?

There are meta-arguments like count or for_each in Terraform that allow to work with multiple resources without creating multiple resource blocks.

I wonder if I can add my own syntax to resources that would do the same thing - make user able to work with multiple resources using one syntactic block but not by using count or for_each but my own directive with different syntax and rules?

This is not possible at this time, sorry. Feel free to open an issue against Terraform to request the feature, though!