Looks like I may have found a function that addresses my needs. Bootcov in
Design handles bootstrapping from clustered data and will save the
coefficients. I'm not entirely sure it handles clusters the way I'd like,
but I'm going through the code. If it doesn't, it looks easily
re-writeable. As far as I can tell, boot in package boot would do clusters
only if the estimation function passed to it pastes together data based on
the clusters boot selects.