Is it the steerer tube on the fork that's tapered on the Warbird, or the headtube/headset in the Warbird's frame (larger bottom bearings, but it'll still take a standard 1 1/8" steerer)?
IF its the latter, try looking at the Soma Saga, Soma Double Cross Disc, the Surly LHT Disc, Surly Straggler. All are steel forks, will take a disc brake, and have mid-blade rack mounts. Might have to call Soma about the Saga fork, as its not listed on their website as being a separate item from the Saga frameset.
Did you found a solution for this problem!.. I have exactly the same dilemma on my warbird ti, I want to fit a rando bag, and the thule rack looks like a good solution, I'm just skeptical to put it on the enve fork... Cheers