I’m working with someone to design a coat of arms and we’ve tentatively settled on this shield.  He’s asked me for the blazon, and I’ve suggested "Sable three chevronels issuing from sinister braced Or, on a chief wavy Argent a lion rampant between two fleurs-de-lis Gules."  Does anyone think there’s a better/different way to blazon the chevronels?

http://www.americanheraldry.org/forums/attachment.php?attachmentid=1366&stc=1&d=1414199000

Is it unusual for a chief to partially cover a charge as it does here?

Very nice!  The blazon looks right to me.

As to the slight overlap of the chief, which in this case doesn’t obscure anything significant below, and actually allows the chevronnels and the chief to be a bit bigger & more visible than if there was no overlap, this strikes me as a good exercise of artistic license.  Maintaining a strict no-overlap approach would result, in this case, in a poorer result.

In other (different) designs the verdict (mine anyway) might well be different - if e.g. the chief obscured or distorted the charge(s) below.  It’s a case-by-case determination in each case.

On partial coverage: suppose it was a bend, or, say, a pile. Wouldn’t the wavy line of the chief necessarily cover a bit of the upper end?

I suppose the artist could draw it either way and be technically "correct" but not allowing the overlap might well (I’d say usually, but who knows?) result in a visually poorer result.

Heraldry is sometimes said to be both art and science - in this case, IMO art trumps.