How about you let the bridesmaids and best men start walking at the beginning and middle of the song? Let there be a gap with each couple for timing. Then when you know the song will be ending, you begin to walk?
I'm doing that, actually. I'm playing Bach - Pachelbel's Canon in D. However, it's not only 6:00 minutes long, but I also don't want to walk down until the upbeat part near the end w/out sacrificing the build up of the song. So, I'm timing it, I even have the seconds down pat now lol At the beginning, the bridemaids w/ the groomsmen start walking, two at a time. A cute idea I decided to do to pass the time was... the groomsmen and bridesmaid stop in the middle of the aisle, the groomsman then gets down on one knee facing the bridesmaid and offers her a rose. The bridesmaid smiles and accepts it, then the groomsman stands, takes her arm and leads her the rest of the way. The next two, then repeat it and so on. By the time it gets to the part I want to walk down, the flower girl goes, then I follow with my dad. And an idea I got from the forum...I will have two detachable roses in my bouquet...if there's still time to pass(so the song ends well) when I reach the end of the aisle, I will stop and give a rose and a kiss to my mother and the same to MIL. Then start the ceremony. If I don't have time, then the rose and kiss exchange will be after we're pronounced man and wife.
That's just an idea...Like you, I was worried about the song ending too abruptly and probably awkward.
I think in tradition, you just stay out of sight...is there anywhere you could hide until you walk down? I know with an open gazebo that would make it harder.