Then, inspiration struck! I went to the fabric store, got a 3 inch piece of 2 inch wide industrial strength sticky-backed velcro. Then I attached the soft side to the buckle and the rough side to her booster seat. I used the other half to do the same for her brother's booster.
Getting-in-the-van time is back down to its normal 5 minutes, and even big brother admits that it's easier to get buckled now!