Extracting Some Subcarriers after demodulation

Hello, 

I am working on OFDM TX and RX using GNURadio and N210 USRPs. The OFDM implementation is already exists and it works perfectly. 
I am trying to pick or extract some of the subcarriers (or multi-tone) to do some phase measurements on some of the extracted subcarriers. 
This should be done after the demodulation. As depicted in the attached figure, it should be after the FFT of the payload and before the OFDM frame equalizer. 
The problem that I am facing now, is how to extract these subcarriers?
Filers cannot be used since the OFDM signal has already been down-converted (baseband). I am working to create my own python block, but still facing a lot of troubles. Can anyone please give me some guidance on how to extract some subcarriers ( e.g, two subcarriers)?

Regards,
Wesam