No, I just don't have any local access to vermiculite, or perlite that doesn't contain Miracle Grow fertilizer, so I just used a handful of the mulch. I like experimenting with incubation methods, and I've successfully done substrate-less with leopard geckos, and I've used rockwool for substrate and substrate-less incubation without issues. As long as I don't start seeing mold or fungus, and I used enough water, I don't foresee any problems.
I'd be confident using just about anything that can absorb water for substrate-less incubation, as long as it isn't extremely perishable. I think I'll be trying a few other substrates this year, so I'll definitely make a post with the outcomes after the season is over.