while working on BLE stuff with nRF52 I looked around for similar platforms that could be supported in NuttX. I looked at STM32WB, which is based on a dual-CPU and supports BLE (as well as various other 2.4GHz protocols). While not as open as Nordic why publishes the details on the low level RADIO, they offer a firmware which runs in one of the cores and includes a full BLE stack, with some kind of IPC mechanism as API. But more importantly, it can be configured to only run the Link Layer from the blob and offer an HCI interface. In this case, supporting it in NuttX would be fairly easy. The downside is that these chips / boards are more expensive (I think 45 usd for the Nucleo shaped board) and are not (yet?) widely available as modules.
There is also the TI CC13xx and CC26xx supported under arch/arm/src/tiva.
