Now that https://github.com/JuliaLang/julia/pull/39455 has been merged, is there documentation on how to use MKL blas via libblastrampoline?
I don’t think so. Note that it has only been merged a a few hours ago. However, you can check out [Do not merge] Use the new LBT mechanism introduced in Julia to forward to MKL by ViralBShah · Pull Request #62 · JuliaLinearAlgebra/MKL.jl · GitHub. AFAIU, eventually MKL.jl will do the job for you.
Just to keep you up to date, you might want to try Run on Julia 1.5 (by building sysimg) and 1.7 (with LBT) in the same codebase by ViralBShah · Pull Request #63 · JuliaLinearAlgebra/MKL.jl · GitHub, i.e. use Julia#master in combination with MKL#lbt2. Overall seems to works great! 