Dimitry Grinberg did the opposite; he used a Nordic nRF24L01+ to fake BLE GATT transmissions. It won't ever connect as a BLE device, but smartphones can see his advertisements.
You could take his research and easily detect the beacons. You won't be able to interact with them since the nRF24L01+ isn't up to the task, but your question stated that you merely wanted to detect them.
To paraphrase his excellent posting:
- BLE advertisements occur only on three channels.
- Data whitening and CRC are done in software
Your microcontroller (a little AVR or possibly even something like an Arduino) could hop between these three advertisement channels, listening for packets. When you think you've found one you de-whiten the payload and compare the BLE destination address to to 0x8E89BED6 (the destination address for unsolicited advertisements).
It's not point and click, but it should be doable.
The Nordic nRF24L01+ modules are available everywhere. Most look something like this:
. They're just simple SPI devices and as mentioned, libraries exist to drive it already (just google links, not endorsing any particular library).
Brennan Ball wrote a fantastic tutorial on using these devices. Highly recommended reading if you're going to roll your own software.