I know this is an important topic but I think that, as with anything in life, it has to be handled on a case-by-case basis.
When I was younger, I would have said, "It's absolutely never permissible to date a separated person, ever!!" and I'm not necessarily saying it's ever right.
It's just that, as one gets older and talks to people about their lives, you come to realize that life is NEVER black and white like we wish it would be.
After being single for so long and on Christian dating sites for several years, I've run into many people whose spouses simply refuse to grant them a divorce, even though the spouse that refuses has, in some cases, moved on to another person and even had children by them. It might have been 15+ years, but their spouse refuses to agree to a divorce.
I don't know what all the absolute answers are. As I said, I would guess it depends on a case-by-case basis with a lot of prayer, counseling, and Christian accountability.
But I would hate to think that someone in this type of situation will be held hostage by a prison of isolation for the rest of their life, all because of someone else's selfishness.