I'm not a technician, I'm a ramper. I routinely service lavs. You've got it pretty much down pat Palms. You've got the access panel, within is the blue juice fill port, the dump handle, and the waste valve(opening, whatever). You first open the access panel(of course), and then open the outer flap(and get the hell out of the way). You then hook up the hose, and then open the inner flap with the little handle next to it. You then pull the T handle and the SH!T FLIES.

Now, I've had plenty of lavs where the T handle was inop, the actual valve stuck open, but the handle stowed and thus able to close the access panel. I've also seen lavs with the inner flap not secured. If both the "sh!t valve" and the inner flap were inop, all the "contents" is right behind that outer flap. That must be opened before you can hook up a hose. You'd jump out of the way, but it splashes(I know, it happened to me ONE TIME). That was an accident, I'm confident in that. But one could flush a rag or something, leaving the "sh!t valve" cracked open, and leave open the inner flap. You then have a nice surprise.

Now, I've not actually seen this happen, but I've heard multiple accounts of multiple incidents involving a local outsourced ground handling company.