The layer covers the entire layout, you could have it do anything you want when you click out there by simply putting an invisible sprite trigger out there.
As far as making buttons so they can't be clicked when the layer is open, in my space shooter in my sig I use two methods for my store menu and options page. I have some that are turned off via variable once pressed, ie in my on clicked event I have it set a variable to "clicked" and I condition out Clicked=0 in initial on clicked event. Then when you want it available again simply turn it back on by setting variable to 0.
If you have many buttons you want to turn off (ie. my entire store is a hidden layer on main layout) just put their event coding into its own group and activate/deactivate the group when the layer is visible.