Video Polarity and Direct Injection

[Electronic M]

Looking at the circuit again...
At the input if you put it before the blocking cap you may have impedance mismatch with your coax, if you put it after you will change transistor bias (which could resort in distortion), if you ad another DC blocking cap you may create a RC filter network (and would have to make sure it does not attenuate in the video pass-band).

Feeding the output of the coupling/blocking cap into your pot/divider seems the most logical idea to me.