  Radial velocity monitoring has revealed the presence of moving broad emission
lines in some quasars, potentially indicating the presence of a sub-parsec
binary system. Phase-referenced, near-infrared interferometric observations
could map out the binary orbit by measuring the photocenter difference between
a broad emission line and the hot dust continuum. We show that astrometric data
over several years may be able to detect proper motions and accelerations,
confirming the presence of a binary and constraining system parameters. The
brightness, redshifts, and astrometric sizes of current candidates are well
matched to the capabilities of the upgraded VLTI/GRAVITY+ instrument, and we
identify a first sample of 10 possible candidates. The astrometric signature
depends on the morphology and evolution of hot dust emission in supermassive
black hole binary systems. Measurements of the photocenter offset may reveal
binary motion whether the hot dust emission region is fixed to the inner edge
of the circumbinary disk, or moves in response to the changing irradiation
pattern from an accreting secondary black hole.
