Art,
This is considered an 'Investigative Video'. You would still charge your hourly rates for both shooting and converting the footage into digital format, but you would not be allowed to edit the clips lest they be subject to challenge in court. Head and Tail Slate, time stamp the video during the shoot and I highly recommend you shoot it on tape so that the raw footage will be available for examination when the eventual challenge of the material comes.
When you digitize, burn a timecode counter onto the clip at the bottom of the frame so the lawyers can easily call up points on the clips. Use a separate external drive to digitize and process the digitized footage. When it's all done, keep the raw footage and the external drive packaged together locked up in a safe or lockable filing cabinet if they lawyers don't require the original material to be in their custody. Most important, is once it's locked up, don't mess with it! Only take it out when it is requested. Make sure you charge a one-time 'storage fee' if you're keeping the original material onsite.