I know this is a really old post, but I just happen be trying to do the same thing (Hide couple expressions in a legend) so I was thinking of creating my own legend as you mentioned. Could just provide a little guidance as how I could try an achieve this. Sorry for any inconvenience
Try to disable the legend and creat a straight table to use as legend. A straight table can be formatted to look like a legend. Add the dimension, 1 as expression and remove totals, the header, the caption, make cell borders transparent etc. The example posted here has such a fake legend hovering on top of the chart.