While this may not be exactly what you are looking for, it may be interesting to compare Jain and Buddhist approaches to meditation: The First Jhana as an assimilated Jain Meditation Practice - #6 by dayunbao
I agree with you that understanding the fluid meditation/religious environment in the time of the Buddha would be very interesting. We often think of Buddhism in isolation, but really it was part of a competitive “marketplace” of ideas, each seeking adherents/followers. The Buddha was successful for many reasons, including but not limited to the general coherence of his religious framework, the visible success of his sangha, and his personal integrity. But he did draw and benefit from the practices around him as well.