I disagree with you. I'm exactly sute how large the market is, but a friend of mine runs a very large landscape business. Besides his hourly employee's he uses day labor as well. Some of his contracts are with daycare companies, school's, church's (most have daycare during the week) etc. He told me that his contracts won't allow him to use people that have been convicted sex offenders and certain other crimes at these type's of jobs. It would be a nice feature if when he was using day laborers he could do an immediate search before he picked them up.
I'm sure there are other situations that this would come in handy, but as I said it might not be a large business but might be worth tinkering with.
As full disclosure I did reg criminalhistory.mobi a few month's ago