Layers can't work that way - they don't have access to the current content item.
What you need instead is Paperclips, which is a feature I wrote to do this kind of work. In fact it's probably far more suitable for what you need, for a variety of reasons.
It's part of the Mechanics module. Basically it lets you create connections from a given content item to any other content items, and then add PaperclipPart to those connectors to define which zones they should get pushed to on Detail view.
It works equally well for pushing both content and widgets to zones, since widgets are just content anyway :) You can also have per-user and per-site widgets or content, by creating connections to the user or site content items. Then there are events you
can hook into (ISocketEventHandler and IConnectorEventHandler) if you need to code any custom display rules, filter the connectors, etc.
Mechanics is available on the gallery (but you also need to install Origami first), or you can get the latest code for both modules and see the documentation at
http://scienceproject.codeplex.com. The latest version has a number of fixes and improvements which will up on the gallery later after a bit more bugfixing.
The documentation isn't complete yet so if you need any help with it just ask :)