Getting a TAI timestamp is impossible because the BIPM (the authority that defines TAI) does not want TAI used for such purposes. Without approval from the authority nobody will undertake to construct the technology to provide TAI to an operational system. Without a source for TAI, any system which claims to be using TAI is making the same mistake that POSIX made with UTC. To wit: Creating a new thing which has the name of an existing thing but does not have the properties of the existing thing. This is a recipe for confusion.
TAI is calculated in retrospect, as I understand it, based on the weighted contributions of the atomic timescales maintained by all of the participating countries. As a result, if you store a TAI timestamp in hopes of using it as an absolute time reference point, you will have to go back and update it when BIPM releases the next 'Circular T' bulletin (http://www.bipm.org/jsp/en/TimeFtp.jsp?TypePub=scale).
