How do I display a selection of products from my network?

Site:
WordPress Multisite
MarketPress
Post Indexer

On my main site I'd like to be able to display featured products from across the network. I know I can display all products using the [mp_list_global_products] shortcode, but I want to be able to choose which products are featured.

Is there a way to do this? I know I'd need to identify the selected products in some way - by adding them to a category (not ideal as the subsite users could mess about with this and add their own products) or by listing the products in a widget that I can then display on the main site.

Any ideas please?
Thanks!