On my upgrade necks this poses no problem, because you can easily adjust the relief without removing the neck. There is another high quality neck available on the market that I also carry. At this time it does not provide for adjustment without removing the neck entirely. If you own already one of these I can modify it for you in about 1 hour's time. Steinberger uses a Phenolic fingerboard exclusively on all of their necks.
Phenol is the prime ingredient in Epoxy.
But understand that Phenolithylin has no memory like real wood. I know you thought your Steinberger neck was totally indestructible. It would be totally unrepairable even if you did have a truss rod it could not be fixed. I keep several of each model in stock at all times. I can also satinize either your existing stock neck or a new stock neck for a small fee, if you feel inclined to do so.