There is no supported firmware ready for the nRF21540 supporting BLE at the moment, as the BLE TX power threshold for what is allowed around the world is +10dBm in most countries (all of Europe).
However, there is support for all recent (post v.3.0.0 at least) nRF52-series SoftDevices for controlling external RF front-ends using GPIOs to toggle TX/RX based on the nRF52 radio state. nRF21540 supports being controlled like this, but you won't be able to leverage the gain adjustment settings in the nRF21540, as SPI control is needed for that part (which is only supported in the Thread and Zigbee SDK as of yet). It might be possible to adjust the gain through SPI if you add this feature to your application yourself, but there are no guarantees.
As far as I know, the nRF21540 should work with nRF52 + SoftDevice, but with limited control support.
Yes, you should be able to just use the RX pin like this by setting the TX pin to GND. Keep in mind that there might be some extra work by controlling the nRF21540 like this, as adjusting the gain to what you'd like won't be as straightforward as using SPI control would be.
