The low voltage relay is probably going to be your easiest way. The 12v. can go across the contacts, and then the coil of the relay is fed from the alternator. If voltage drops the coil out the contacts close and the red light comes on.
Some one wire alternators have an internal jumper from the one wire to the internal regulator and can be opened up and that lead tapped to attach a wire to an idiot light, but it might void your warranty on the alternator.|
Another method is to get a std. GM 3 wire alternator and jumper it to make it a one wire, then use terminal #1 to feed the idiot light. In these alternators you have a single wire off the post to the battery to charge, then there are #1-#2 post, and a jumper from #1 to the bolt on post makes it a one wire, and #2 is the indicator.