Find v13.2.6.0 - Unified Search - Storing Lists as part of SearchMetadata

Hi All, 

I've ran into a problem that I'm hoping someone can help me with.

I've written a unified search solution, but need a way of storing additional metadata to items I'm surfacing in search, that will be used to filter against, and build facets from. 

The information I need to store against the unified search items are originally entered via properties decorated with SelectMany attribute (essentially a collection of checkboxes in the CMS UI, stored as a comma separated string)

[SelectMany(SelectionFactoryType = typeof(EnumSelectionFactory<T>))]
        [Display(
            Name = "Document Type",
            Order = 50)]
        [Required]
        public virtual string DocumentType { get; set; }

I then want to ideally store this information as a List<string> but there is nowhere on an ISearchContent type where I can assign the value - there is only one property on this type thats of type IEnumerable ("Categories" - ref: https://world.episerver.com/documentation/developer-guides/search-navigation/NET-Client-API/searching/Unified-search/), but I will need to store many lists of strings - not just the one example above, but up to 6-7 lists potentially. 

I then need to create a FilterBuilder and match on any of the values in the stored string - by conventional means there is no DelegateFilterBuilder I can see that would allow me to do this - there are obviously ones that work on collections, but I can't get the content into the Find index as part of the SearchMetadata as a collection! 

https://world.episerver.com/documentation/developer-guides/search-navigation/NET-Client-API/searching/Filtering/Strings/ 

It looks like my options are limited with ISearchContent - can anyone advise where to go from here? 

Is it possible to create my own DelegateFilterBuilder that splits the stored value and then filters based on that? 

Thanks!

Paul