  The recent detection of a transient absorption feature in the prompt emission
of GRB 990705 has sparked multiple attempts to fit this feature in terms of
photoelectric absorption or resonance scattering out of the line of sight to
the observer. However, the physical conditions required to reproduce the
observed absorption feature turn out to be rather extreme compared to the
predictions of current GRB progenitor models. In particular, strong clumping of
ejecta from the GRB progenitor seems to be required. Using detailed 3D
hydrodynamic simulations of supernova explosions as a guideline, we have
investigated the dynamics and structure of pre-GRB ejecta predicted in various
GRB progenitor models. Based on our results, combined with population synthesis
studies relevant to the He-merger model, we estimate the probability of
observing X-ray absorption features as seen in GRB 990705 to << 1 %.
Alternatively, if the supranova model is capable of producing highly collimated
long-duration GRBs, it may be a more promising candidate to produce observable,
transient X-ray absorption features.
