I saw this twice last week, with no call in either case. Just wondering...

Ball four on the batter, ball gets away from the catcher. The on deck batter bends over, picks it up and tosses it to the catcher. The runner is still trotting to first. No harm, no foul?

Weak Interference
 

This sounds like "weak" interference to me. You need to adjudge whether the contact by the offense created a situation where the defense was put at a disadvantage. If so, kill it and make it right. If not, play on.

I would just hold the BR to only first base and let things be.

And then tell the new batter that he just prevented his partner from reaching 2nd - scoring position for his upcoming hit.

Well, not really... but it would be fun.