Thanks for sharing your solution, I'm sure it will be useful to other users who run into the same issue :)
I shouldn't expect an alpha channel to cause an issue with an imaging API - I imagine that would be a big problem. It might be worth issuing as a bug to tcod on their git repo.
The png is exported at 4bit by default (not entirely sure why) - though looking into it I could probably crunch it down to 1bit. I'm not sure if that will remove the alpha channel or if that will have a negative impact on other implementations.
How did your experiments go?