i3: Can I temporarily force any new window to open floating?

In the case where I know a window is about to open, sometimes I want it to open normally (making other windows resize to make room), but other times I know I'll want it to float and avoid making things resize. One such scenario is when a lot of terminal windows are open, where resizing them will lead to their display getting messy, and I'll only briefly want the new window. I don't want to configure rules per application or window class for this, at least not for the purposes of this question. What I'd like is to be able to hold a key as I'm expecting the window to open, and if the key is being held the new window will be forced to float, so other windows do not resize. Is such a thing possible? How might I achieve it?