Is there a good way to remove hard-coded subtitles from a film?

The first thing you would need to do is ensure that both sources are exactly the same running time - you may need to trim frames from one of the sources to get them both to line up frame-for-frame.

You would then need to upscale the laserdisc source in such as way so that the picture information in each frame aligns with your HD source to the nearest pixel. You may also need to tweak the contrast, brightness and colour to get the sources to match as best as you can.

The next step is then to overlay the LD video over the top of the subtitles. The simplest way would be to use a constant rectangle over the region of the subtitles, but this may end up distracting due to the quality difference in the videos, and if there is any significant telecine wobble in either of the two it will probably look awful.

A better result can be obtained by using third video that contains a mask for the subtitles. It may be possible to create the mask automatically if the subtitles are easily distinguishable from the video, for example if they are bright white and the rest of the video is darker, but in all probability you will have to create your mask video manually - and this means a lot of work if the subtitles appear all the time!

The advantage of using a mask is that only those parts of the LD video that lie directly over the subtitles will end up being overlayed on your HD source, so it's potentially less distracting. You could also feather the edge of the mask to blend the two sources together and avoid any hard edges.

The above process is based on my experiences with AviSynth, but there may well be a much neater workflow with a pro editor like FCP, After Effects, Vegas etc.