Thank you for your suggestions but it does not fit this “use case”
I typically test multiple hypothesis using the same model (models of factorial designs) using linear combinations of the parameter estimates.

What I actually looking for is a recommendation for functions which would help me to interpret and visualize the results of

postsample <- posterior_samples(fit1,"^b")
as.matrix(postsample) %*% cbind(c(1,1,0,-1), c(0,0,-1,0 ), .... many more )

Ideally I would like to be able to use exactly the same functions which can be used to summarize brmsfit models e.g. summary.brmsfit.