Numerous reports have been made of features, in either emission or absorpti
on, in the 10-1000 keV spectre of some gamma-ray bursts. Originally interpr
eted in the context of Galactic neutron star models as cyclotron line emiss
ion and e(+)-e(-) annihilation features, the recent demonstration that the
majority of gamma-ray bursts (GRBs) lie at cosmological distances makes the
se explanations unlikely. In this Letter, we adopt a relativistic fireball
model for cosmological GRBs in which dense, metal-rich blobs or filaments o
f plasma are entrained in the relativistic outflow. In the context of this
model, we investigate the conditions under which broadband features, simila
r to those detected, can be observed. We find a limited region of parameter
space capable of reproducing the observed GRB spectra. Finally, we discuss
possible constraints additional high-energy spectral observations could pl
ace on fireball model parameters.