Hi, just chiming in since the topic is of interest to me 
First I want to say I truly believe Mobilizon can help with the mission of having decentralized events and I think we’re at a special moment for it with users demanding such a platform. Meetup is rumored to cover a 2$ fee per RSVP soon by the way.
I agree, and we can’t overstate how vital it is to maintain a good climate between the sometimes very different communities in our strive for destroying (Internet) monopolies.
But also it seems it to be a quiproquo: because their goals are different and complementary, starting from one’s code or the other’s should not be a topic, to my understanding. Instead, reinforcing both Mobilizon and Demosphere’s ecosystems mutually is indeed the common goal, as already underlined.
I agree a custom integration ends up being the obvious solution, although maybe some import/sync features could maybe put in common.
So, I was trying to imagine what a custom integration could look like between, for example, Mobilizon and Demosphere (or Demosphere/Mobilizon and Radar, although I don’t know Radar as well). Couldn’t we have a special public Mobilizon instance called the “Demosphere Mobilizon instance” that would pull and parse Demosphere’s RSS/ICal feed? Would Demosphere be opposed to this idea?
Regarding ICal being “pretty rubbish for forcing a format for the location”, Demosphere’s moderation already seems to enforce a quite standardized location format for each event.
This doesn’t solve the fact that developers resources are scarce and that maintaining these bridges probably takes a lot of time and effort.
What isn’t clear for me is: will Mobilizon include importing events from ICal at some point? 
I only saw the “Export to ICal” feature in the code at the moment. I still think this “import” feature would be helpful, even if it’s offered as a plugin or as a quick script.
