I have found a paper on a generalization of the Birkhoff-von Neumann theorem here:

[http://cowles.econ.yale.edu/conferences/sum-09/theory/che.pdf][1]


  [1]: http://cowles.econ.yale.edu/conferences/sum-09/theory/che.pdf

Here is the Abstract:

The Birkhoff-von Neumann Theorem shows that any bistochastic matrix
can be written as a convex combination of permutation matrices. In particular, in a
setting where n objects must be assigned to n agents, one object per agent, any random
assignment matrix can be resolved into a deterministic assignment in accordance with
the specified probability matrix. We generalize the theorem to accommodate a complex
set of constraints encountered in many real-life market design problems. Specifically,
the theorem can be extended to any environment in which the set of constraints can
be partitioned into two hierarchies. Further, we show that this bihierarchy structure
constitutes a maximal domain for the theorem, and we provide a constructive algorithm
for implementing a random assignment under bihierarchical constraints. We provide sev-
eral applications, including (i) single-unit random assignment, such as school choice; (ii)
multi-unit random assignment, such as course allocation and fair division; and (iii) two-
sided matching problems, such as the scheduling of inter-league sports matchups. The
same method also finds applications beyond economics, generalizing previous results on
the minimize makespan problem in the computer science literature