I am not sure, that is a good question. People have shown that it is possible to hack a nRF24L01 into sending BLE transmissions. But there are limits. It could be possible to sniff parts of BLE traffic.
That's pretty cool. I always looked at the rtl-sdr stuff but lost interest when I looked at the max frequency being less then my signals of interest. The MMDS apparently changes that though, didn't know about those before.
BUT: This has been done years ago for under $1 (the price of a nRF24L01 board) by using a nRF24L01 itself.
http://travisgoodspeed.blogspot.de/2011/02/promiscuity-is-nr...