fixed observation-level variance in glmm

Dear mixed modellers

I have a data set classified by two factors A and B both of which are
random. There is also an observation-level (residual)  random term that has
a known variance,  defined by a known constant cv.  Since the dispersion
parameter in a Gamma glm is the  cv, I want to do something like:

glmer(y~(1|A)+(1|B),  family=Gamma(link="identity"), dispersion = cv  )
where cv is known.

I can not see this facility or syntax in any of the mixed model tools I
know of. Is there an easy way to fit this model?

Thanks
Ric

Richard Coe
Principal Scientist - Research Methods
World Agroforestry Centre (ICRAF), Nairobi, Kenya
and
Statistics for Sustainable Development, Reading, UK

Phone: +447734104196, +358503247733