In this scenario why don’t you link to a keyword search rather than linking to a theme result as it’s possible to assign multiple keywords to products. Just be sure to use terms that are not going to result in organic matches to any of your product data, i.e. avoid words like wooden, black etc and separate the words with commas; mpwspc,pr5ad89 from the below examples.
As an example, here is our specials page with products assigned the keyword mpwspc
Products contained in that search can also have additional keywords assigned so they can be grouped into smaller segments specific to the individual promotion, similar to what you do currently with your themes.
Here’s a subset of products that are also listed in the above search.
Hope it helps.