How much higher of an oxygen supply would I realize and possibly benefit from
in close proximity to plant foliage, such as laying on grass or
sitting under a tree(or trees), factoring out variables like
geography, species, or wind currents?

> How much higher of an oxygen supply would I realize 
> and possibly benefit from in close proximity to plant foliage....?

Not much.  The effect can be estimated indirectly, by considering the
diurnal variations in CO2 concentration of the air due to
photosynthesis.  Such variations have been measured in many locations
(see references in http://www.med.harvard.edu/chge/qrsummer02/Tubfull.htm),
and are due to the effects of photosynthesis during the day, and plant
respiration at night (as well as decay/oxidation organic debris).  The
maximum daily "swing" in CO2 concentration is on the order of 100
parts per million by volume (ppmv).  This is a rather large relative
change in CO2 concentrations, amounting to a ~25-30% change relative
to the average atmospheric CO2 concentration of ~370ppmv.

The CO2 that is removed from the atmosphere by photosynthesis is
consumed in a chemical reaction that looks like:

    CO2 + H2O -> O2 + CH2O

The O2 is released to the atmosphere, and the CH2O represents part of
a glucose molecule made by the plant. For each molecule of CO2
consumed by this reaction, one molecule of O2 is released.  That means
that if the CO2 concentration in the air is reduced by 100 ppmv, the
O2 concentration is increased by 100ppmv.

The total oxygen concentration of the atmosphere is about 20.95% or
209,500 ppmv, much higher than the CO2 concentration.  A change of
100ppmv in O2 concentration is only a 0.05% change in the relative
amount (100/209500 * 100), which is a very small change.