How to fix individual elements of the residual covariance matrix in MCMCglmm

Hello list,
I am using the MCMCglmm package in R for mixed-model multi-response regression, and I am trying to fix individual elements of the residual covariance matrix (when using rcov = ~ us(trait):units). I thought it might be possible using the 'fix' argument in the prior, but it seems that this either fixes the whole residual matrix to the prior specification, or allows all elements to be estimated. This fixes the whole residual matrix:
prior <- list(R = list(V = diag(5), n = 6, fix = 1), G = list( G1 = list(V = diag(5), n = 6)))
I believe 'fix = 0' would have the same effect as not specifying 'fix' (i.e. the whole matrix is estimated).
Is there a way to fix individual elements (e.g. I may want to fix the covariance between two of my five response variables to a specific value)
Thanks,
Richard.