If a MACHO passes over the face of the star it is lensing, then one can measure the MACHO's proper motion, as discussed by gould94 and nemiroff. When the MACHO passes in front of the stellar disk the shape of the photometric curve differs from what it would be with a point-like source star; the peak is flattened. Observation of a single event of the kind will give an upper limit for the MACHO's proper motion, relative to that of the star, if one assumes that the MACHO crossed centrally over the stellar disk of known angular diameter. Thus, a statistical analysis of several events is required. The unknown angle between the directions of the two proper motions also gives an uncertainty in the interpretation, but this is probably small since MACHOs are expected to have much higher space velocities than the source stars.
For a Galactic halo MACHO of 0.1 
the Einstein radius is 0.2 mas
when observed at a source of V= 16 mag, according to HNP-Table 1.
A typical source star at the median distance 6 kpc has the radius
mas. Therefore a fraction of 0.002/0.2 = 1 percent
of the photometric events would contain a stellar disk crossing.
The crossing of the stellar diameter at a velocity of 200 km/s takes
about 4 hours.
To make a clear measurement one should probably sample the light curve
of order 10 times as the MACHO passes the stellar diameter. This
implies a desirable sampling interval of 
hours.
If GAIA rotates with 120 arcsec/s the observations by the three
directions of view will be spaced at about 1 hour intervals, and some
12 such observations by incoherent imaging
would be obtained before the band of scan of 1.6 degrees width has
moved away from the star. This seems realistic enough to warrant further
consideration in connection with GAIA, in view of the importance of
improving our knowledge of the dark Galactic halo.