Extension of BDQ's accessory code, mainly adding new partials for the front end
See paulcc's fork of spree-demo (http://github.com/paulcc/spree-demo/tree) to see this code in actual use
Edit the product accessories in the admin interface (look for the new sidebar tab), and then include or adapt the products/_accessories partial in your site.
quantity selection for accessories with variants does not work yet: either needs some JS to tie things together, or a tweak to the product/variant/quantity representation
however: most of your accessories might be simple items without variation...