It is not clear to my why you believe that lowering body surface pressure (everywhere except over neck and head would have a meaningful effect on venous return
Other than single patient long term experiences in iron lungs there has been few if any "studies" after 2002 listed in several data bases. A further search was unable to find a current manufacturer for these devices.
Having cared for several patients in "iron lungs" in the 1960's I can attest that it appeared that during the negative pressure phase (inspiration) it appeared that cerebral blood flow (CBF) would decrease marginally. (I say appeared because during that period of time measurement of CBF was pretty difficult requiring invasive measurers with significant risks to the patient.
I had a situation where I transported a patient with a PEEP of 20; A little outside of my comfort zone. My thinking was that external negative pressure could assist right heart filling in patient s with a poor ejection fraction, and also improve alveolar exchange by “pulling” rather than “pushing”. Just a thought.