The Wellcome Trust has released Developing an Effective Market for Open Access Article Processing Charges.
Here's an excerpt from the announcement:
In their report, published in March 2014, Björk and Solomon set out a series of scenarios for how funders might develop their approaches for supporting APCs. These cover both full open access journals (which operate exclusively by this model) and so-called hybrid journals (which offer this service for individual articles, while continuing to operate via the subscription model). The authors appraised three combined scenarios, which they conclude to be the most promising for further consideration.