Here is a not necessarily complete list of players registered by MLS clubs during the Secondary summer window, who have already appeared in games for their new clubs. This excludes Americans and Canadians (even if transferred from foreign league) and at least 2 players who previously played in MLS.
Aaron Boupendza, Gabon, Cincinnati
Eugene Ansah, Ghana, Dallas
Teemu Pukki, FInland, Minnesota
This can be done.
It has been 360 days since Taty left, 202 since Heber, and 178 since Alex Callens officially departed. And their replacements aren't in country yet.
I would assume -- and this is just an educated guess on my part -- that Miami (and these other teams) started the process before making the official announcement. Messi was obviously happening for a long time before the official announcement. For whatever reason, NYCFC seems to wait on the visa until the official announcement is made.