Can I clean a second story dryer vent that goes out the roof from the inside?
My experience using those brushes is that it's difficult to tell if you're pushing on a lint blockage, a turn in the pipe, or your knocking the cap off the end of the pipe. So if at all possible, you should have the end open and/or connected to a vacuum. You don't want this tool ramming into the motor in your dryer, nor do you want it knocking the damper off the outside of your house. In your case, that's going to require someone on the roof.

You can get close, keeping track of how many segments of the cleaning tool you used to get to the roof in a previous cleaning, and then stop a few feet early. But as Michael says, the cap itself should be cleaned since lots of lint will build up there.