Make the guides:

To make an object into a guide is really simple.  Just select the object you want to be a guide and from the View menu go to Guides->Make Guides.  It’s default keyboard shortcut is command-5 on the mac.  Poof – you have a guide in the shape of your object.

Tie that in with offset path and you have a really speedy way to build margins. As long as they are not locked, you can use them with the align panel and smart guides.

The minor issues:

Even though it shows as a guide, (that is, just a stroke) it can have a fill. That might come in handy in some cases, but so far has only annoyed me as I try to click through them. Easy enough to remedy, just set them to no fill (which can be done even after the objects are converted to guides).

Every time you hide and then show your guides (command-;) it locks them.  That means you can’t select them to use with the align panel.  Easy enough to remedy, just hit command-option-; to unlock them again.