You won't get a "portrait" type result with on camera flash, but you can get good results... especially if the images won't be used for anything special... which it sounds like they won't. A couple of tips to consider, because I've gotten totally hosed by people "providing a backdrop" in the past. Most non-portrait photographers have NO IDEA what a backdrop is, and what can or can't be used. I've spent hours before trying to make a backdrop work that the customer provided. If they are doing this, I would go there ahead of time ... not on the same day... and test what THEY are providing. I would not agree to use what they are providing until I had seen it in person.
When you say "backdrop", are they talking about an actual backdop... like a painted muslin... or just a "background" that would work. You may find almost anything. If they are providing an actual photography backdrop, then they may be expecting studio type lighting which you'll have a tough time replicating with a single on camera flash. Or they may not.
If you are shooting against a backdrop... a wall... etc., then make sure you pull the subject out from it at least 5 feet or so. This will avoid your getting shadows on the backdrop which are hard to correct in processing and really take away from the image. Then you'll need to make sure you have enough light to expose the background properly, and let the ETTL take care of fill on the face of the subject. This will also introduce some difficult light since the temperature of the backdrop lighting and subject will be different. I'd try to do a custom white balance if you can, or you may get some odd results. One of the benefits of actual studio strobes is that you generally overpower the ambient light so your light is purely one temperture... on the subject and the backdrop.
If their idea of a "backdrop" is really a scene or something, then make sure it is lit the way it will be lit when you take the pictures... when you test. I got burned at a high school dance by this. They had this beautiful indoor scene with plants, a waterfall, bridge, etc. They wanted this to be in the background of their posed shots. I spend well over an hour trying to get this lit up as it was kind of dark. I had a lot of strobes pointed at different angles out onto the scene which went out pretty far behind the subject. I had it looking pretty good and then about 1 minute before showtime, someone flipped on additional lights that totally hosed up all my pre-planning and I immediately had a line of folks ready to get their picture taken. I was so annoyed.
So just remember that they may actually recommend to you, or give you something, that is impossible to use well.