While @MerakiDave is most likely correct (he's great with getting us answers here on the community!) the "official" way to get these numbers is to simply call support and ask. They'll happily provide you with MTBF numbers for any Meraki device.
My first approach was to open a case on Cisco Meraki Support. But the reply was "Unfortunately, this is not something that support can provide. You should be able to get this information from your account manager (help -> Get help)".
Since it was urgent (just needed that to close the answer to an RFP) I have used the MTBF indicated by MerakiDave.
@Jbarb just closing the loop on this, I circled back with the MR team and the MTBF values have been added into the data sheets maybe a week or two ago.
MR36: Mean Time Between Failure (MTBF): 174,300 hrs at +25°C operating temperature MR46: Mean Time Between Failure (MTBF): 1,406,652 hrs at +25°C operating temperature MR46E: Mean Time Between Failure (MTBF): 1,328,360 hrs at +25°C operating temperature MR56: Mean Time Between Failure (MTBF): 1,315,498 hrs at +25°C operating temperature