I'm certainly no expert on this, but I think that the "isopiestic"
technique could be usefull for you. As I understand this can be used to
determine water potential in small soft tissues, like leaves, that are
otherwise difficult to measure using the pressure bomb technique. The
isopiestic technique is described in this paper:
J.S. Boyer and E.B. Knipling, Isopiestic technique for measuring leaf
water potentials with a thermocouple psychrometer, /Proc. Natl. Acad.
Sci. USA/ *54* (1965), pp. 1044–1051.