>>169 >>171 Voice work is usually saved for the completion of principle photography.
The only possible option here is a solo greenscreen scene which I'm positive the director would prefer to be present while filming with his trusted crew and own direction. I doubt they flew over his Loki costume and wig to shoot one scene which they could shoot in Oz.
I think he was exaggerating at CC when stating how desperately he had to be back on set. CC was organised way before any film scheduling and was most likely built into his original contract. If he doesn't have a hectic shoot schedule (and I doubt he does) it's not that big a deal to give him the whole week off.