MCMCglmm Bivariate model and prior

Dear MCMCglmm users,

My questions are simple and I hope you can answer me.
I wish to run a bivariate model with MCMCglmm package. But one response 
variable is a continuous one (family = gaussian) and the other is a 
binary one (family = ordinal).
I have sought a way to parameterize the model (in the archives of this 
list too), I tried some solution that I thought, but in vain. I have to 
fix to 1 the residual variance for the binary variable and leave it free 
for the continuous variable.
Is it possible in MCMCglmm ? And if yes : how do I parameterize the priors ?

Thanks for your help.

Gr?gory DANIEL
PhD Student - LBBE
University of Lyon 1
69100 Villeurbanne - FRANCE