All pilots are trained for this manoeuvre in the simulator
I'd like to see the full-motion simulator which can accurately emulate the sensations of a barrell roll.
Seriously, yes, a proper barrel roll can be flown within the structural limits of most aircraft. The 707 video is proof of that. BUT if you get it even slightly wrong you may end up inverted, nose up, with nothing on the clock with the makers name, or in a steep dive requiring many g's to recover. In both cases you WILL exceed the limits of the "normal" or even "utility" category.
So do me a favour and DON'T TRY THIS AT HOME unless you are, or have been, under proper aerobatics instruction in a proper aerobatics plane.
Namibfox, the Aresti symbol you supplied is actually for a slow/aileron roll. A barrel roll doesn't have an "official" Aresti symbol (it's not an aerobatics competition figure) but the informal notation is to add a little circle to the tip of the arrow.