Very interesting question. The challenge is techincal relaibility I fear. I have found an article in the grey literature describing an USS technique used to measure CVP based on the pressure required to collapse the internal jugular. Do you think this might work?

Conf Proc IEEE Eng Med Biol Soc. 2006;1:3843-9.

Ultrasound-guided noninvasive measurement of a patient's central venous pressure.

Aggarwal V1, Chatterjee A, Cho Y, Cheung D.

Author information

1Dept. of Biomed. Eng., Johns Hopkins Univ., Baltimore, MD, USA. [email protected]

Abstract

Central venous pressure (CVP) is an important physiological parameter, the correct measure of which is a clinically relevant diagnostic tool for heart failure patients. A current challenge for physicians, however, is to obtain a quick and accurate measure of a patient's CVP in a manner that poses minimum discomfort. Current approaches for measuring CVP involve invasive methods such as threading a central venous catheter along a major vein, or tedious physical exams that require physicians to grossly estimate the measurement. Our solution proposes a novel noninvasive method to estimate central venous pressure using ultrasound-guided surface pressure measurement. Specifically, our device works in conjunction with an ultrasound machine and probe that is used to visualize the interior jugular (IJ) vein below the surface of the skin on a patient's neck. Once the interior jugular vein is located, our device detects the pressure on the skin required to collapse the IJ and correlates this value to a central venous pressure reading reported to the operator. This quick and noninvasive measurement is suitable for emergency situations or primary care settings where rapid diagnosis of a patient's CVP is required, and prevents the need for further invasive and costly procedures. The measurement procedure is also simple enough to be performed by operators without extensive medical training.

PMID:

17946583

[PubMed - indexed for MEDLINE]

You can directly cannulate the vein with the smallest vein cannula and connect it to a transducer. It will show you the exact pressure inside the vien. 

However, I don't understand why do you want to measure the pressure inside the vein that is used for bypass. The pressure should be more or less very similar with the systemic arterial pressure if there is no proximal arterial stenosis. 

You can connect you arterial pressure line on a small subcutaneous needle and after calibration you stuck the need in the venous graft (some surgeon use this needle for de-airing of venous bypass grafts) and you will get you pressure line on the scope. However it is probably not an option for arterial grafts.