I've not seen it practiced as one long form
But yes, Dahong is in three sections. It would be a monster to practice as one big form. I imagine some people practice it that way, but not me. I barely have the stamina to make it through the first form nowadays (I doubt I even remember it :o ).
i've not seen it practiced as one form either.
but its the same as the three roads to xiaotongbeiquan. they can be strung together as one or practiced as three separates. and i believe it was originally one form divided into different roads. again, the first of which is generally known or taught.
a similar thing happened to daluohanquan. its 108 movements long. it was cut in half and an ending was added for closure. most people will tell you thats the whole of it but there's more. but its not generally known or taught either.
You could draw a parallel to the Yang Tai Chi long form
Yang Tai Chi has three distinct sections too. Many people just practice the first section and end it there. That's what I do sometimes, mostly because I can't remember the rest. When practiced as a whole form, there's a sort of a pause point where you could bail out at the end of the first and second sections. That blends into the next section if you choose to finish it all. Good question about hero's pose. Would you practice it or not if you strung them all together? I wonder what the original transition might have been. I'm not sure it makes that much of a difference now. I'm sure it's very subject to interpretation, since so few people practice it this way anymore.
Actually, when it comes to stacking forms, you can practice however you like. It's your practice. If you're not experimenting with it, you're not really practicing it. You could string xiaohong and dahong together into one massive form. Back in my BSL daze, we used to string the short five forms together into one long form, just to work our endurance. It wasn't something we did very often, but it served to mix it up a little when we were feeling unusually spunky. We'd often mix and match forms like that - go from one to the other - just for more training.