Let me address some of your concerns.
but didn't go further in detail here. So, essentially, one idea to reduce the drop-out rate, while make it less of a sunk cost, is to offer it as a protection fee: When you sign up, you have to pay amount X as a deposit. IF you show up, you can chose to donate it to OTS (e.V.) or get it back. That way you'd pay for not showing up only. Considering that General Assembly asks for 55£ in London for a three hour afternoon frontal-course intro into HTML/CSS (random example), I think it isn't too much to ask for a deposit, if someone wants to take a workshop with us to secure their seat (because then others can't). And all non-shows are directly donations to the local or global OTS.
As per diversity support, we can have a simple policy for that case here, too: non-securable seats, free from deposit fees – we keep X% of seats to be assigned only. Similar to diversity/community tickets at conferences, you can get those seats by sending a message explaining why you deserve those seats to the organisers, who'll put you on those spots. Doesn't have to be much, but requires you to take more effort and direct communication than just pressing a button – hence making it much more likely that people actually show, as they now know that the organiser knows them. But the extra effort makes more sense, when people would have to pay otherwise ...
Fair enough. I am totally up for having some kind of ploicy how that money, deposits or charges, are too be allocated. And we can say that a certain X% have to go to global operations, while another part stays with the local budget to run the events – as per mentioned posters and flyer for events are always local.
Which requires time. Time that someone needs to spend on doing that. For that we often have the policy of saying, "hey, if you want to do that, yes, let's go." But then don't back them up with anything. Not even flyers to be able to give community centers something into their hands, that they even start talking with them.
If this were easy, why hasn't it been done?
Just for the record, to organise the Rails Girls Summer of Code I understand they have two-full time paid staff. This is were time becomes money.
But in general, we aren't the Rails Girls. We don't have a community of enthusiastic, rich companies advertising their technology behind us. Our tweets aren't retweeted a couple of hundred times by core-committers to the framework, language and posted on blogs. We don't have companies lining up, paying for stuff. They do. There is plenty of theorie, how they got there and why they have such a reach today, but that doesn't matter to the fact that, we are not the same. We do not. We don't have that reach and when we propose sponsoring to companies, we have a much harder time. Taking that into the account that there is very little time people are able to spend, it is a very clear explanation, on why this hasn't been done more.
But even if we did, it'd only solve the issues for us here in Berlin. That is not a solution to places like Dortmund or Rhein-Main, where there simply aren't these types of companies, nor do they have connections to make that happen.
Although I do think that Berlin can improve, I am not talking about trying to solve problems in Berlin, but to offer a tool for solving them everywhere. Leaving it to the local group whether they want to use them or not. At the very least, the global organisation should acknowledge these problems and offer an officially blessed structure to tackle them. Whether the Berlin team uses them, is up for a separate discussion (I think it should!).