Hi Moore:
This was the subject of an interesting discussion at the 2013 TDA Summit. The Association tried to codify your scenario and several others related to situations where "chips are added to a bet not yet pulled in". However, when the delegates were polled on various situations the votes were all over the place: "it's a call", "it's a raise"...
At the end of the day, instead of adopting a set of rules addressing each possible scenario, the more general Rule 44 was adopted:
44: Previous Bet Chips Not Pulled In: If a player faces a raise and has chips in front of him not yet pulled in from a prior bet, those chips (and any change due) may affect whether his betting response to the raise is a call or re-raise. Because several possibilities exist, players are encouraged to verbally declare their bet before putting out new chips on top of chips from a prior bet not yet pulled in.
The good news is that your example is one of the few situations where there is near-universal agreement: it's a call. Why? A) the previous bet chips have no change coming from the previous bet (in this case the SB); B) the prior bet chip(s) don't equal a call of the current bet (the BB); and C) the player adds a silent single overchip. Under these conditions it's no different than any other silent overchip bet (Rule 42). Also, some TDs are using Rule 49 "Non-standard and unclear betting" to rule that the action is the "lesser of" the possibilities in cases where there's doubt...
NOW, there is at least one twist to your example: what if the guy puts out the 500 and then "tosses" both the 500 and 100 forward... is that a call or raise? There's just so many possibilities with these add-on bets that writing strict rules for all of them may never work... and even if we did, would dealers know all the variations? Hence Rule 44: "players are encouraged to verbally declare their bet before putting out chips on top of chips from a prior bet"...
I've slightly changed the title of the thread as there are quite a few questions on this topic. Thanks again for the great question.