It might be a bit cost prohibitive to do now but a 1% grade probably wouldn't be noticeable and BB's should roll off to one corner to be collected in drain pipes or similar. I know that some indoor places that use "reballs" do this and basically just sweep the reballs off to a corner for a spray down, drying out and reuse. For the record, reballs can be used like 600 times over and over according to the FAQ on their webpage.
But then again to do a 1% grade now you'd probably need a building permit and building inspector to come in and look at it. That is unless there's already existing grade for water drainage in which case the BB's will just collect there and you can just sweep them up after the fact. Or just "pressure wash" the BB's while you're spraying down the floor and push them to a corner for easy scooping up.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE