Yes, not as crazy as you think. Pretty mainstream in our bassclass with P.V.E.You use your thumb for the G#, one half-step above the open string?!? We're talking about Simandl half-position, right . . . and you're using your thumb for that note?
He would demonstrate Mozart 40 last movement in thumb as one fingering option. Bb-D-C-A-Bb-D-C-A in tumb on the G string, and that is the one I never could get to work for me in real life. I blame a smaller hand and inferiour shifting technique. (thumb on all of the A and Bb!!)
I also use 4-2-thumb and 4-1-thumb sometimes in lower registers. Whatever it takes to get the job done. If you practise low thumb scales and get comfy, it can be a very useful tool for certain problems.