Once again, so much more than meets the eye! Wow. This has been a very interesting thread to read, because honestly, all the times I've seen BBM it had never even occured to me that maybe this was a missed opportunity for a deeper, verbal exchange of what they were feeling. Or that maybe this time it might have been Ennis who wanted to say more, if the groundwork had been laid right by Jack. Will have to look for that next time, see if I get a change of perspective.
It's very true that this is one of those few moments that WE are given to maybe witness them speaking of their feelings. As you've all noted, there are other missed chances before or after this, but since this was also one of the few points where they could have changed the course of their lives, it does really stand out. And it hurts us because that doesn't happen. As Jack_ME says, we're left with "what if ..", "maybe ... " "might have... " "sigh".
I guess for the moment, personally, I too think that Jack is very much aware of what is really going on in Ennis's mind. He opens up the opportunity for conversation by asking Ennis if there is anything interesting up there in heaven. And even though Ennis makes it into an inside joke (they were always kidding about that harmonic on Brokeback, so sweet) Jack surely knew what that answer was really covering up.
"for you forgetting to bring that harmonica" translates to "for you being here".
And on a lighter note, (it's not my intention to offend anyone here joking about this important scene but ...) when Jack says "you know it can be like this, just like this, always" we all know he's talking about them being together, not that they will always be "harmonica free".

Although, maybe Ennis is hoping for that, too.
You know, I just realized, these boys make it interesting for us because even when they
are talking, we still wonder about what they are really
saying .
I was right there with you, Jack_ME, that first time I saw BBM, 'cause I was waiting for the guys to actually SAY more about their feelings for each other in this scene, not just with looks, touches, body language. But being who they were, I guess I can admit now that I had an unreasonable expectation, based on MY needs. Didn't stop me from being disaapointed, though.
Sorry to ramble on .. looking forward to reading more about how the rest of you feel about the possibilities in this scene. Great posts!