Not with the cumulative family due to the order requirements of the thresholds. Other ordinal families support that via adding a (cs(1) | item). Note that I don’t like the current category specific (cs) syntax for varying effects and might change them to cs(1 | item) at some point.
A new paper by (http://dx.doi.org/10.23668/psycharchives.2725) proposes partially pooled thresholds for the cumulative family and a generalization of this method may find its way into brms eventually.