Disclaimer: I have never built a homemade bator.
However, I have read quite a bit about world traditions for incubating, and it's quite amazing the different ways people have of hatching eggs, especially without electricity. Did you know it can be done with rice bags warmed in the sun?
So, anyway...... I suspect that if you can get the incubator up to the right temp using heat tape (which is GREAT stuff by the way--used to have it for our ball python, along with a SUPER nice VERY expensive thermostat scaleable for multiple reptile habitats but ALAS I let it go with the snake when I gave him to my friend, lol), then it should work fine. In fact, heat below is great because heat rises... and I've seen many homemade bators with a lightbulb in the bottom--makes sense to me--seems like you'd have fewer temperature layers that way.
As for using it in the brooder, I've wondered the same thing. A mother hen, of course, would provide heat from above. But I don't know that it's necessarily necessary. Interested to hear what others say about it.