Focusing on how or whether orks progress the state of their technology kind of forces us to question how orks know things, I think. Everybody seems to know about Ghazghkull, for instance, but ... how? Large as his Waaagh! may be in terms of sheer number of boyz, it isn't very large in astrographic terms, so it can't be that enough orks have rotated through his Waaagh! for veterans to spread the news by word of mouth (and in any case, if an ork found himself in the biggest Waaagh! in history, why would he leave?). Gargants are a similar problem: everybody seems to know about them, but the codex tells us that the original idea came from one identifiable mekboy. How does a race with little to no directed space travel spread ideas like that?
Whether it's the Waaagh!, space hulk express, or some other mechanism, it's clear that actual ideas get transmitted among even very far-flung ork tribes. I'd put forward this transmission of ideas, rather than the transmission of schematics, as the mechanism for what little orky advancement we see (and I do think we see some). I don't think that mek A builds a gargant and then transmits (whether by Waaagh!, space hulk, black box, or whatever) to mek B, "Hey, here's how you build what I just did!" Instead, I think what happens is that mek A builds a gargant and mek B hears about it (again, however it is that orks hear about interstellar news), and thinks to himself, "Wow, what an incredibly orky idea! I bet I could build something like that if I tried!"
Every now and then - very rarely, I admit - an ork seems to come up with a brand new idea or incremental advance that seems to appeal to the ork psyche. The gargant is one of them, which presumably has such widespread appeal for religious reasons. The shokk attack gun seems to be another. Ditto with large-scale teleporters. But the critical quality those things have is, I think, that they seem particularly orky (as opposed to particularly effective, for instance). If ork technology was going to advance from its current state, I think it would be in that way - somebody would come up with a machine that embodied some aspect of orkyness in a way that nobody had thought of before, other meks would hear about it, and be inspired to try their own hands at creating something similar.