Allow Signups to work with Entities

As it stands currently, Signup is limited more towards being a standalone "event" signup system. I know of at least two use-cases where Signup, in its current form, is a good starting point, but doesn't quite reach where it could.

Use-case 1: event booth-space booking and registration (see http://drupal.org/node/1279538).

Use-case 2: product registration and information attachment. In this case, I want to have multiple products be signup enabled, but each one has its own data that needs to be attached with it. IE: I have an instructional class vs a gym pass, with an instructional class I want participant info and guardian info; whereas with a gym pass I only need participant info. Signup currently attaches these fields to the user profile, and shows those fields on every signup, regardless of what signup "type" it may be.

I'd like to see Signups be entities in and of themselves. That way we can create multiple signup entities, each with their own fields, and attach these signups to other entities (nodes, user, commerce_product, etc).

This would definitely solve my use case (#2), in that since I use Drupal Commerce, I can attach a signup to my commerce_product entities. As it stands, I can only enable Product Display's (since they are nodes) to have signups, which is not exactly what I want. I don't want a user to sign up for a product display, especially if that display has multiple variants of a product (gym pass, gym pass with locker, gym pass with bigger locker, etc). I have no way of distinguishing that the user signed up for Gym Pass with locker rather than just Gym Pass without having to load the commerce order and checking the line items for a particular product.

Proposed changes:

• Signups should be entities in and of themselves
• With signups as entities, they would be fieldable and completely customizable for each user's distinct needs, and would no longer be attaching non-user specific fields to the user profile
• Signups should have the ability to be attached to other entities, not just nodes (hook_entity_alter)

I realize it's quite a big structural change, but I think it'd be infinitely more available to users who may otherwise pass over Signup for not meeting their needs.