Most of the examples that I have seen using foam rollers (those semi-hard cylinders) target the muscles in some way: they are promoted as ways to strengthen the body, mostly the core it seems. That is yang-yoga and could be great. But can they be useful in yin?
Perhaps! Like working with balls (from softer tennis balls to hard rubber balls) these could be used as massage devices. Remember the principles in yin yoga however: long held, static stresses - but never to the point of pain. There are many possible ways to use foam rollers to help increase a stress on the connective tissues or deepen the stress while in certain poses. While I have not found it necessary to go there (most of the time, I get enough juice just as it is, thank you very much) for some people this may be a fun and effective way to do their yin practice.
Try it out! Let us know how it goes.
Cheers
Bernie