How does strength of a pulse related to EKG(ECG)?
Higher QRS complexes particularly in the lateral leads would suggest hypertrophy of the left ventricle. This in turn would suggest a stronger pressure is in place, although it is not known whether this is because it is required (incompetent valves) or a physiological response to exercise (where the volume is likely to actually be bigger). 

Alternatively, if it really is just displacement of the brain you're interested in, why not use an EEG taken with an ECG and measure if there is a variation with each heartbeat. Then you can tease apart whether this is due to pressure or oxygenation or many other things. 

Otherwise you could place something overlying the carotid which will loosely, but perhaps more accurately, reflect the pressure and one could potentially calculate the volume this way once enough other factors are known.