More experimenting using maps to organize Susy grids (or any other kind of data). Essentially, this is just abstracting calls to map-set() and map-merge to make the syntax more friendly. Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.0.rc.1)
// Compass (v0.13.alpha.10)
// ----

$_grids: ();

@mixin add-grid($g, $name) {
  $_grids: map-merge($_grids, ($name: $g));
}

@function grid($name) {
  @return map-get($_grids, $name);
}

$grid-1: 90%  8 1/4;
$grid-2: 100% (4 8) 1/4;

@include add-grid($grid-1, 1); // Add grid-1

a { output: grid(1) }
b { output: grid(2) } // Will output nothing since it hasn't been added yet

@include add-grid($grid-2, 2); // Add grid-2

c { output: grid(2) } // Now it is output

SassMeister-output.css

a {
  output: 90% 8 1/4;
}

c {
  output: 100% 4 8 1/4;
}