Custom Context

Does anyone have experience writing custom context conditions into modules? I have one that has me stumped.

What I need is a simple context, the plugin for which gives the user a short list of allowed values (3 or 4) to be compared against a single external variable (in this case, stored as a session variable).

I have the rest of the framework I need pretty much hammered out at this point-- it's just the custom context that's causing me heartburn.

Any takers? I'm happy to provide more details upon request.