Experimentation ... experimentation... like Thomas B and Matt above have suggested. I can't really suggest anything for that combo, but I've found this photographer who has gotten it right:
http://www.flickr.com/photos/zgodzinski/3447881393/ - pushing HP5 to 1600 and developing with R09-rodinal for 24 min, at 20C. I dont know how he did it without lowering Temp.
http://www.flickr.com/photos/zgodzinski/3443036818/ -same here.
He looks like a very accomplished photographer, and he latter changed to TriX-Rodinal combo.... exceptional work...