Positronium splitting?
To my understanding, positronium has an average lifespan of around 140 ns before decaying, but can the positronium atom be “split” into its constituent particles, say with a strong magnetic field, before annihilating?

Why the downvote?  A sufficiently energetic photon should do the trick (theoretically), but whether an experiment is possible or not is another matter.

The lifetime of positronium depends on whether it is in the singlet or triplet state (para- or ortho-positronium). The former has a lifetime of about 125 ps, the latter, as you stated, around 142 ns. source: wikipedia
The binding energy can be calculated to be about 6.8 eV - so a well-aimed photon should be able to split positronium. Experiments to measure hyperfine splitting have been reported - this suggests that the EM interactions of positronium behave as expected.
