Thanks, David; this is useful information, and certainly sheds light on the problem. In particular, you are correct that the Standard GUI does not generate inaccurate bitmap plots; I had overlooked this fact. The following sequence generates a good plot in xmaple, but suggests a root near x=0.5 in command-line maple:
The fact that "command line Maple isn't intended for plotting" would appear to be the whole of the problem here; I can certainly try the MapleTA forum, but I think I know what I will find there. From what I can tell, the MapleTA plot generation functions *ARE* the command-line Maple drivers; in both cases an 'mplotgif' process is spawned, and the output is visually indistinguishable. If it is indeed the fact that command line Maple is not intended for plotting and if this necessarily means that command line Maple cannot produce full-featured bitmap plots, then the chance that a workaround exists for MapleTA is essentially nil.
This is why I posted in the general Maple forum, and I think I have my answer; again, the information you've provided is greatly appreciated. If anybody else has any other creative solutions, I'm all ears.
Command line Maple output for above: