I use my road bike now but I’m a big believer in train with the bike you race too. And I also reach for the rollers versus the trainer in most cases. I do enough outdoor workouts on my MTB that I don’t need to sweat the specificity of the different positions drop bar/flat bar. I think that if you’re doing early season work indoors on your road bike, that’s ok, but if you’re more concerned about getting as much time as possible on the MTB position (makes sense if it’s new to you) then it might be worth chasing this.
I suspect you’ll be able to do at least sweet spot work with MTB gearing and no resistance unit, but that’s just a guess. If you’re looking to do threshold and above it’s likely that resistance will be needed. I’m a little bummed that my Feedback rollers will be limited to my drop bar bikes, hadn’t checked until this thread!
And if I may, while I do appreciate the need to train indoors on the MTB, try to get outside as early and often as possible. I’ve found more success with outdoor workouts than indoor. Depending on your local terrain I recognize that this can be challenging, I’m lucky that I live in the mountains and have found a suitable sustaining gravel climb that I do most of my work on. Just as rollers are so much more dynamic than a trainer, performing the same work, but outdoors is the next level for specificity. I get it if that’s not possible and I’m only mentioning this as I’ve learned this lesson a couple of times after spending months indoors, only to feel flat outside when the time finally comes to express that hard earned fitness in the real world. Forgive the aside.