Many hotels allow you to leave your bags in reception prior to checking in. This sentence appears on the hotel's website:
"Hotel guests may leave their luggage with the bell desk during their stay. There is no fee for this service."
That suggests that you might be able to leave your bags there as soon as you arrive in Vegas.
Additionally, this appears on the website (which, incidentally, gives the check-in time as 4pm):
"Early check-in and late check-out are a possibility upon arrival/departure, based on room availability"
Once again, it seems that the hotel is keen to assist travellers who arrive or leave at unusual times.
For confirmation, send them an email:
http://tinyurl.com/58vlyj
(Make sure that you mention the Rio in your mail, because that web form directs mail to the group's HQ).
Alternatively, simply phone them up. From the UK, the number is 00 1 866 746 7671
Chris