The best solution here would have been N- development, so I would try to reshoot if possible. If you haven't worked out film speeds and development times for N- development for your preferred film (sometimes it helps to increase exposure with shorter development times to support the shadows), it would pay to run some tests first or bracket and make duplicate exposures, so that you can adjust development time after viewing the negs.

I haven't done much flashing at the print stage, but that is a technique that Les uses, so look at his book to see what he says.

Alternately there is contrast masking, if you've got the registration equipment.