I don't think you're understanding the situation. Mahler calls for the two hammer strokes to be doubled with bass drum. If you don't use a bass drum, in most instances, there's going to be absolutely zero reverb to the hammer sound itself. Now, these days, many orchestras have built these big wooden sound boxes. In such cases, you probably don't need a bass drum stroke. But if you're hitting a big wooden hammer on to some sort of wooden chopping block, you really aren't going to get all that much sound UNLESS you electronically amplify it. Further more, the second hammer stroke gives the conductor the option of adding cymbals and tam-tam (and most conductors do pick up that option).
So, the bottom line is this: regardless of how giant of a wooden hammer you might be swinging, it really isn't going to make a huge enough noise against Mahler's 'wall of sound' coming from the brass, UNLESS you also have some kind of huge wooden box with a sound hole to go along with the hammer. In most instances, the bass drum is really needed.
Also, keep in mind that there's nothing in Mahler's annotation in the score that says that the hammer should completely bury everything else. Mahler was more concerned about the quality of the sound than the actual volume. He makes it very clear that it is to be non-metallic, but doesn't really specify wood. It's really a logistics problem, and there's only so much insisting that ANY conductor can do with the Vienna Phil.