Thanks everyone for your suggestions.

The weather forecast for this weekend looks terrible, so I think I am going to delay my start by a couple of days. Even though I ordered that bumper bar, I think I am going to use the extra time to try one of these DIY solutions. The reason is that even though it seems right now that pushing the bag forward is all that will be required to keep it off the tire, I'm worried that the bag will start to sag once it has had some road use (as djb has pointed out). A solution that supports the bag from underneath seems like a reasonable way to ensure that doesn't become a problem. The way the tent bag is made, it is the poles, rather than the tent fabric, that are exposed to potential damage, and they are likely to tolerate more abuse, but I don't want my nice things destroyed in any case.