Best method / hints on digitising a rolled up panoramic photo

My first thought is that humidity is your friend, at least as far as rolled photographs are concerned. One approach is to put the rolled photograph into a closed container with some damp cloth/paper towels/sponge making sure that the photograph itself does not get wet. After a day or two see if it's a bit more flexible.

A suggestion from the National Archives is to carefully unroll a short segment of the image (imagine a scroll) at a time, allowing it to curl before and after the small area you've flattened. Then photograph that narrow flat area and composite the series of photographs into a panorama.

A combination of the two approaches may be your best bet, but the re-humidification may do the trick on its own. Don't leave the print in the container too long (multiple days/weeks) so as not to risk mold/mildew formation.