Like vandannen said in his answer, it is possible if you use move scratchpad
on an entire container.
But some problems (crashes) I experienced, when starting to modify this floating container, including moving windows outside of it, lead me to believe that this is not an intended or at least well testet feature.
I also doubt that this will ever change. The only reason i3 even has floating windows is because there are some applications that do not work well with arbitrary window sizes and dialogs.
Scratchpads are essentially floating windows with some extra functionality for hiding and retrieving.
An alternative:
Not exactly what you want, but you can have multiple scratchpad windows, they will be shown alternately on scratchpad show
: show A -> hide A -> show B -> hide B -> show A ...