I like the idea of cutting the film in half and develop each half according to how it was exposed.
The only way to lift shadow values on the EI 250 'half' is going to be to develop the film longer. Much longer. It's in these situations that I consider standing development to be a very good choice - an extreme technique for a pretty extreme situation. Use Rodinal at 1+100 dilution. Make sure you have at least 3ml of Rodinal concentrate in the mix. Prewet the film. Agitate for the first full minute, and then let the film develop for about two hours with such underexposure. If it were me I'd probably agitate every 30 minutes. It's a long shot to get it to work, but if you develop those frames normally, you aren't likely to get anything on them anyway. What have you got to lose?