How are you going to show the slides? Are you burning the slide show to a DVD to play at the funeral? Are you playing them on a computer or connecting your computer by firewire to a monitor?
If you share it to iDVD for burning to a DVD disk, there is a "Loop" selection on the Advanced menu, that you can access after selecting your slide show title box in the main screen menu or in map view. I don't know any way of playing the slides in random order.
Anything that you export to QT can be played in loop, by selecting Loop in the view menu. But, again, no way to show the slides in random order.
When in iPhoto, there is a way to play like a slideshow photos that are in an album. There, you can select "repeat slideshow" and "shuffle", and then the photos will repeat in some sort of random order on your computer. But I don't think that those settings are preserved if you export to iDVD or Quicktime. So, you would have to firewire your computer to a monitor or TV to play the slideshow in random order by playing an album as a slideshow.
Possibly you can buy a plug-in that might do a random display, maybe Photo to Movie or Photomagico. You would need to verify that those programs had a random playback feature.
If I were doing it using iPhoto, I would duplicate my string of photos a couple of times in the iPhoto slideshow, and manually scramble the duplicated strings so that they appeared to be randomly displayed. That is, the strings would be in different fixed orders, but they would appear random when played strung together.