Oscar,
Severely underdeveloped means low contrast. Shadow detail should be less affected, however. Check your shadows to see if there is detail there. Even if the underdevelopment slowed the film way down, you need to build contrast. Here's what I'd try:
First, even before trying to print with grade 5, I'd bleach and redevelop in a staining developer. That means using a rehalogenating bleach (I use potassium ferrocyanide and potassium bromide), and bleaching the image away. The image silver turns to silver bromide, which can then be redeveloped. Using a staining developer like PMK for the redevelopment will add an additional stain image to the silver one there already, thereby increasing contrast and adding a bit of detail to the shadows. I've used this with good results in the past. I know you don't have bleach, but the above works well and is easy. Read up on it if you are interested.
Toning the negative in selenium toner will give you about half a grade of contrast increase. It will also make bleach-redevelopment impossible, so it's one or the other, since toning a stained neg in selenium removes the stain... Anyway, KRST 1:2 for about 5 minutes should do the trick.
Then, grade 5,and the contrastiest light source you can find. If you're using diffusion, try a condenser enlarger. Use your Dektol straight. All this will help you build contrast.
If none of this works, maybe it's not worth it. If it is, then I'd go to hybrid next...
An interneg is also a possibility. You can contact print onto film or paper and see what you get by varying development times. Unfortunately, if there is no shadow detail, you really can't save what's not there.
Hope this helps some,
Doremus
www.DoremusScudder.com