This needs to be implemented. Sometimes I just don't want to invest a huge chunk of time to a game, but it'd be really cool to hop in on a 5v5 going on and chat and compare strategies/learn while doing other things irl.
While we're on the subject, why do the new versions keep making the old replays obsolete? It's very anti-community imo. These are huge opportunities that are just being thrown away.
If old replays are important to you- PLEASE make a copy of your sins of a solar empire installation directory. Rename it to reflect the version is a good idea. This isn't an insult as there are many of us that do EXACTLY this after we learn the first time or two. This approach also works with mods.
I can't really see how the developers could make a replay from a previous version work when the underlying code goes through significant changes.
Note: not all patches fubar replays and save games. It's only the ones that significantly modify the source such that actions that occurred in the replay or save game are no longer valid.