Yes you can. Ive done it many times. Just make sure they arent too big. Sometimes people really pile that icing up and they come out super thick. If your FBCT is half an inch thick its probably going to fall off the cake. Keep it thin and when it comes time to put it on press it gently into the side of the cake. Sometimes Ive even put a little blob of BC where Im going to put the transfer and then piped a little border around the FBCT to help secure it to the cake. Ive not had one fall off yet.
I did frozen buttercream transfers on the sides of a 49ers cake that I made. They didn't fall/slide off. Like TexasSugar said, make sure you press them on really well. I had a few spots where mine didn't want to press in quite all the way so I went back and piped a little buttercream the same color as the edge of the FBCT behind it to fill the gap to make sure it adhered well. It worked pretty well. Here's the link in case you want to see it:
Thanks for your posts. I made one and put it on the side of the cake and so far so good. I am a little worried about delivering it in this heat, but as long as I crank the ac in the vehicle I should be okay.