I have never seen this done in brms, but I might have missed something about brms.
About modeling correlation parameters: this seems pretty uncommon. I once saw it done in an IRT context in this paper, where they used the Fisher r-to-z transformation as a sort of link function.
I am also reminded of this post by @mike-lawrence, which describes difficulties in accurately estimating correlation parameters. That sort of issue would seem to make it more difficult to do what you want to do. If you only had categorical covariates like itemtype, maybe it is possible to estimate a multiple-group IRT model to do something similar to what you want.