That is exactly what you can do, make your VM repo on GitHub and test your changes. You can keep everything updated throught the SVN. If you have good solutions, instead of sending your pull request, create a patch for the SVN.
Same thing, just another language.
I use GitHub for other projects too. Many projects are on GitHub, so I use GitHub. If a project is on a self hosted SVN, I use SVN. I am comfortable with both.
And you are right, in a little team, SVN has advantages, but it doesn't matter if it is public or private. The VirtueMart SVN is public, everyone can see the progress on dev.virtuemart.net. You can take part in discussions in the forum. And if you show real interest you can get in contact and chat with the other devs and testers, these chats are kind of private, but everyone can ask to join or some are just invited.
The project is having regular devs and testers for years, since Max took over. There are people that don't like the way, how restrictive Max handles things, but there are also some who like the restrictive way for a good reason. "Adding a feature should not break another." Not everything gets into the core, but if you see the huge step, VirtueMart did from the first VM 2 versions to the latest VM 3.x, there is no doubt that the way is right to keep a stable core. And that is the most important for shop owners. They need to rely on a stable shop. That was not perfect in the past, but is becoming better and better, so less adjustments have to be made from update to update.
If you have a good template, there is mostly no work involved to keep the shop running like before.
What you said about democracy, VirtueMart is not a democracy, it is a community project, but the last word is with Max, he is the lead developer and responsible. That might be a problem for some people. I know at least two from reading the forum.
Take Joomla, hosted on GitHub, as an example, Joomla is not a democracy. There is a company that holds the trademark. The community is not deciding on the features. Open Source is not equal to democracy. Open source means open source, nothing more, nothing less.
From that point VirtueMart is maybe even more a democracy than Joomla, because Max gives great value to what others suggest if it is prepared properly. If you throw something in front of him, that you worked on for two hours, and Max has to invest another five hours to make it work, don't even bother. I made that experience myself in the beginning. I offered some code, and most was rejected, rightfully as I understood later.
But learning from experience I prepare my mostly simple stuff properly now and things work out, because the work load on the lead dev is minimal and he can concentrate on more complex things.
Well, enough said for today, some will understand, few will not, and very few just don't want to.