  The optical afterglow of Gamma-Ray Burst (GRB) 000301C exhibited a
significant, short-timescale deviation from the power-law flux decline expected
in the standard synchrotron shock model. Garnavich, Loeb & Stanek found that
this deviation was well-fit by an ad hoc model in which a thin ring of emission
is microlensed by an intervening star. We revisit the microlensing
interpretation of this variability, first by testing whether microlensing of
afterglow images with realistic surface brightness profiles (SBPs) can fit the
data, and second by directly inverting the observed light curve to obtain a
non-parametric measurement of the SBP. We find that microlensing of realistic
SBPs can reproduce the observed deviation, provided that the optical emission
arises from frequencies above the cooling break. Conversely, if the variability
is indeed caused by microlensing, the SBP must be significantly
limb-brightened. Specifically, greater than 60% of the flux must originate from
the outer 25% of the area of the afterglow image. The latter requirement is
satisfied by the best fit theoretical SBP. The underlying optical/infrared
afterglow lightcurve is consistent with a model in which a jet is propagating
into a uniform medium with the cooling break frequency below the optical band.
