If your firewall has NAT46 capability it is possible, but of all the possible NAT things you can try, that direction is the ugliest. The "right" answer is to dual-stack the edge networks, but that is presumably a long-run solution for you, not a short run one. I don't think RDP embeds IPv4 addresses in the content, so it can probably work. In general, doing protocol translation between IPv4 and IPv6 in either direction on things that aren't simple 1:1 TCP connections is a bad idea.
-- Jim Leinweber, WI State Lab of Hygiene