Thanks for the suggestion, but I'm afraid I'm still having trouble...
This is the command line I've set with your suggested modification -
asteroid -artpath "/MAME Support Files/artwork" -autoframeskip -beam 4.00 -cheat -cheatpath "/MAME Support Files/cheat" -crosshairpath "/MAME Support Files/Crosshairs" -flicker 0.00 -mouse -rompath "/MAME Support Files/Roms" -samplepath "/MAME Support Files/samples" -skip_gameinfo -nosleep -snapshot_directory "/MAME Support Files/snap" -video opengl
I then click 'OK', and the error dialog pops up informing me
"Error: unknown option: Support"
I may simply be missing something obvious (wouldn't be the first time...
), and I'm less concerned now that you're aware of the problem.
Incidentally, one of the reasons I'm playing with this option is that the slider in the 'Beam Width' section of 'Core Vector Preferences' still only goes from 0 to 1, incrementing by 0.01 (it should go from 1 to 10, incrementing by 0.1).
The 'Flicker' slider should go from 0 to 100, incrementing by 1.0.