Dear "King of AutoPuzzles Wendax",
absolutely no discussion about your extraordinary knowledge in this puzzle-group. However, I doubt about some of your answers. Regarding your last ones:
1) "All we know about this vehicle is the modified Glas picture above (which is not just an allegation).": I agree with this answer, adding: "...in the moment"
2) "There is no mention at all of a Yugoslav licence fabrication of the Goggo Lastenroller in any books about Glas I know. And Glas has a rather well-documented history. I am not an expert in Tomos history, but the few web pages dealing with Tomos history (including prototypes) I found do not mention it either. These are the facts."
Sorry, I cannot derive any "fact" from this personal statement. You just confirm, that you haven't found anything about it.
3) "... But I still have my doubts that this idea ever materialized into an actual production while you tend to follow the idea of its existence."
Sorry, wrong interpretation. I never have philosophized about existence or non-existence. As, in the moment, there is no evidence about that, except this single (altered) photo!
4) "PS. I'll try not to take the "People who more closely deal with vehicle history may know" personally even though it sounds like spoken from a pedestal."
Please have a look at "2)"
Maybe, a special book could bring more light in the "Tomos threewheeler"-discussion:
Anton Stropnik, "Vzpon in padec Tomosa v sliki in besedi" zaloba Royal Corporation d.o.o., Črniče, 1994
Translation: Anton Stropnik's company-biography "The Rise and Fall of Tomos in Pictures and Words" published by Royal Corporation d.o.o., Črniče, 1994