Jean: I tested your idea and it works well (as I was expecting). There is only the need of tweaking the spacing of points. The resulting curves are somewhat bold but you can adjust the point size to account for that. Of course you need to be able of generate the points and you need to generate a lot of extra points. In my little example 4 times more points.
Tom: Easy done just stacking each matrix with its reverse. Of course as you anticipated if the family of curves don't start in the same point then ugly lines connecting each trace are plotted. In my case i was able to suppress those lines just adjusting a little the lower limit of Z axis. The lines are bolded a little and again you can adjust the line width to correct.
Jean indirectly suggested me another solution for this case: adding some extra points to "route" the fly back outside the clipping box. This works in my case 'cause no trace starts and ends inside the clipping box.
Stuart: I was unable to see the sheet you suggested. My version of MC is old, ver. 2001.
Saludos,
Al