Oh yes, the so-called "Cheater Bar". (I do not care for that name at all given that most rules allow for their usage).
Normally I would say check with the "rules guru" i.e. Derby Committee Chairman or Cubmaster, but it looks like you have those 2 covered already.
Given that, I would run it past your other committee members ahead of time. If they agree it is acceptable, you have greased the skids in case of other parental objections and cannot rightly be accused of having acted as a "law unto yourself". I would also have your book available just in case.
If they do not agree, you can act accordingly, but you must be prepared to deal with the situation if another boy shows up having utilized the same idea. Your committee may decide to rule them out for next year, but I do not like last minute exclusions.
For my 2 cents, it seems OK if it is not excluded by implication of another already existing rule.
Having said that, I have 2 concerns with using a paper clip for this construction:
One is the issue of tripping the light sensor that Mr. Slick mentioned above. When we have used these in the past, we colored the bar black with a Sharpie pen and attached a little piece of tape as he stated above just for good measure.
The second is the durability of the clip at the stop section, esp. if you have (like we did at Pack and Council) a big piece of foam under which the cars get "sandwiched" at the end of the track. We scavenged and adapted the copper alloy (I think) bar from your basic Victor snap style mousetrap for my son's Batman car (see Batman i.e. ...
) and it held up very well. I don't think a paper clip would have survived one round with our Pack stop section.
EDIT: FYI (word of warning) a post was just added click here
that mentions some poor kid who's bar broke during the finals and couldn't be repaired in time.
Mr. Slick is right again that most start sections are executed (mechanically or by hand) quickly enough that you gain little advantage with the bar. We normally use them for figures (like Batman) where the basic design leaves a gap in the middle of the front where the starting post should hold back the car. Side benefit - built properly your car can stand vertically just for kicks!!!
EDIT: p.s. For an example of how they can work with manually operated gates, check out the pics in Gameday's Rail Riding Rail Cars