Interesting. It seems to me that the infrastructure for this already exists so for any particular use case this could be implemented in userspace using inotify and the flag IN_ACCESS. Presumably tmpwatch and tmpreaper could use a mechanism like this listening for files in tmp (tmpwatchd?) or perhaps a daemon could be written which you could use to request atime like information be collected for a particular directory heirarchy. There would be the potential that mutt could use the same mechanism.
One nice property about this solution is that reads being writes are now explicit and if disk runs out read isn't going to fail but the failure mode can be implemented in the watching daemon.
Potentially you could take the dconf like design where a convenient atime API is provided such that atime can be read synchronously by mapping the atime "database" into the process that is reading it read-only whereas the atimed process would be the only process with write access to this file.