First I tried playing the "THX Optimizer" on a DVD player connected via s-video to my capture card, and adjusted the settings so that the captured images were almost identical to the images copied directly off the DVD. However, my laserdisc player's output is darker than that from my DVD player, so the actual caps were coming out too dark.
Then I tried a video calibration laserdisc (see this post) and set up the capture card with that. However, the disc was NTSC (I don't know of any PAL calibration discs) and again the captures from my PAL laserdiscs were too dark.
So I resorted to what I should have done in the first place; capture some scenes from the actual discs, and adjust the brightness/contrast settings until the darkest pixels (in the actual picture, e.g. Vader's cloak - the black bars seem to be darker than the main image) are coming out as black and the brightest (e.g. the roof lights in Tantive) are white. The waveform monitor in the colortools vdub plugin is very useful here. Assuming that the levels remain consistent throughout the disc, this is the best way I think to get things right.
MeBeJedi's philosphy was to black set a bit higher and white a bit lower, to capture all the detail possible, then to adjust the settings during post-processing. His thinking may hold some weight, but it makes more sense to me to capture the full range and leave alone rather than capture a reduced range and then extrapolate.