Answer:

You can pass custom classes to menus.

<md-menu #menu="mdMenu" [overlapTrigger]="false" class="my-full-width-menu">

Then you can target that class with global styles.

For your needs, unfortunately, you'll need to know some information about where your menu overlay is positioned, and hardcode some repositioning

.mat-menu-panel.my-full-width-menu {
  max-width: none;
  width: 100vw;
  margin-left: -8px;
  margin-top: