PRNG taking advantage of very large seed
The PRNG proposed by Barak and Halevi should be able to meet your needs and provide sufficient security. 

Their PRNG has an API of next and refresh. refresh takes an arbitrarily long string and uses it to update the internal state of the PRNG. next returns some number of random bytes (if more is needed, one can simply call next in succession).

They prove some very nice properties of their PRNG (which together define robustness of a PRNG), and give a concrete implementation.

The entire paper is definitely worth a read.