Limiting random results set with facets.

[David Lukowski]

I'm looking for a way if possible to run a query with random results, where
I limit the number of results I want back, yet still have the facets
accurately reflect the results I'm searching.

When I run a search I use a filter query to randomize the results based on
a modulo of a random seed. This returns a results set with the associated
facets for each documentType.

"response":{"numFound":377895,"start":0,"docs":[]
  },
  "facet_counts":{
    "facet_queries":{},
    "facet_fields":{
      "documentType":[
        "78",374015,
        "3",3021,
        "2",736,
        "1",41,
        "34",41,
        "35",32,
        "72",8,
        "7",1]},

How do I limit the number of results returned to N and have the facets
accurately reflect the number of messages?  I cannot simply say rows=N
because the facets will always reflect the total numFound and not the
limited results set I'm looking for.