Unable to fit glmer with family=binomial(link=identity)

Fellow R-users,

I need to estimate the "risk difference" (in epidemiological 
terminology) for a neighbourhood-level exposure (e.g. presence of parks) 
with a binary outcome adjusted on 5 or so covariables. Study 
participants are clusted by neighbourhood .  I was hoping to use a 
generalized linear mixed model with the code:

mod  <-  glmer(outcome ~ exposure + (1 | neighbourhood), 
family=binomial(link=identity), data=rec)

but the identity link is not available for the binomial family with 
glmer (or for glmmPQL).  Is there any way to do this with lme4 or is 
there another package in R which could fix my problem?

I'd be very grateful for any help.

David.

David Evans
UMR-S 707 Inserm - Universit? Pierre et Marie Curie - Paris 6
Facult? de M?decine Saint-Antoine
27, rue Chaligny
75571 Paris cedex 12