Hi lordsnipe,
You need to use a custom line like this:
monitor_specs0 15625-16200, 49.50-65.00, 2.000, 4.700, 8.000, 0.064, 0.160, 1.056, 0, 0, 288, 384
The remarked value is the lowest number of lines allowed for interlaced modes. By default it's 400, that's why your mode was being discarded.
Now be aware you're going to get a very odd aspect ratio there, the same one as 192p.