A while ago I forked DoubleDouble, and implemented exactly this. GitHub - perrutquist/DoubleDoubles.jl: Extended precision arithmetic for Julia
I did this mainly as an exercise, to learn about types, conversion, promotion, etc. Recursive types turn out to be trickier than one might expect…
I more or less abandoned the project for a while because I decided to focus all my efforts on finishing my thesis. (In hindsight, this was the right decision.) I might get back to it now, because I do need high precision in one of my work projects.