Having actually fucked around with this...
It
is very much possible to use alternate routes.
Here's the caveat:
The HUD does not update properly
after setup is complete.
I'm failing to remember the exact conditions, but I believe the path would only properly update the HUD if the route moved from
no roll zones to one with roll zones, and back. Things also get wonky if the route changes in length.
What I did for my
Dynamic Payload contest entry is a bit of a hack. Simply, I set the default route to be path_start to path_end, then have the alternate on path_start to be the true next path, then I can do whatever modifications to the route I want to, then on setup end do the following (IIRC):
1) disable the team train watcher entity
2) switch path_start to it's alternate route (the true route)
3) re-enable the team train watcher
4) profit
If you'd like the uncompiled map to dissect, I'd be happy to supply that.