THIS HAS NOT BEEN TESTED EXTENSIVELY
It works well for a project I need it for. Your mileage may vary, consider yourself warned.
Installation
Install script into ScriptUI folder, a quick google search will help you with this if you don't know how.
Usage
Open the Onion Skin Buddy panel via Window > AE_OnionSkinBuddy, select layer and choose your options.
Number of skins is how many onion skins before/after you need. 4 will create 4 skins before and 4 after. Opacity is the starting opacity for a skin, opacity gradually decreases for each skin created. Skin every x frames is how many frames to stagger between skins.
The workflow is basic, click "Make Onion" to create skins. After you make a change to your layer, click "Refresh Onions" to refresh the skins.
Hope this helps someone else!
The idea for this came from a post long ago by Paul Tuersley on www.aenhancers.com, big thanks for the idea. http://www.aenhancers.com/viewtopic.php?f=11&t=871