I'm trying to develop a way for site users to select a specific product as part of a bundle on checkout by typing in the product SKU.

Example: Users can buy a hat, with the option of bundling with any four buttons on the site. The user should be able to specify which buttons, out of maybe nearly a hundred available in the store. I figure the appropriate time to collect the user's choices would be on checkout, so that the user is asked to specify the correct number of button choices depending on the quantity of hat with button products/bundles in the cart (so there is a field item for each choice to be entered). I decided to not record the user selection on add to cart in the case that bundles are removed from the cart so as to not deal with arbitrarily deleting product choices.

I've been assuming whatever I build will involve some rules, perhaps the commerce bundle module and custom line item types, probably editable fields and possibly some way of adding panes to the checkout form (via a contrib module or code). Also, I've been toying with the idea of loading a VBO view in an overlay via a link in the custom line item to browse available options and update the line item based on the selected choice by passing through some variables.

To make matters more complicated, there isn't a convenient rules event for when a user begins the checkout process, so I've been using the event when an order is updated with a condition on order status, though clicking to checkout from the default cart summary block doesn't change the status of the order.

Any ideas/suggestions would be greatly appreciated. I've hit a road block with most of the ideas I've had so far but still have some others to try.