What works for us is "status meetings" where we all go over a bit about what we are working on for the next few days. They seem pretty useless on paper, but it allows people to chime in with "wait, will you be touching system X", or "hey i worked with that before give me a call", and it gets i'd say 70% of the benefits of hallway conversations, but it's still not perfect.
I feel like a "sidebar" system of "not quite isolated, but not conversation-dominating like talking to the whole group" kind of in-between mode in group video chat software would be a godsend, but the UX of getting it right would be really hard.
Also, for something more programmer related, having peer code-review is really helpful as well. We don't do it as much as we should, but having a few people look over your commits, and you over theirs is a great way to get some of the benefits from that "hallway conversation" while also solving some other problems.
I was recently in the office for a week. I heard so many personal discussions about things, I was able to weigh in on.
I walked away realizing I really am not contributing as much as I could, not being there in person.