I think in the legend you could say: Background = background1 or tempbackground1 or background2 or tempbackground 2 (for example) and this should allow them to be defined as background tiles.
The "temp" isn't a special keyword or anything, but instead "tempbackground1", "tempbackground2", etc., would be whole other objects that would need to be defined within the Objects section (they can look like anything since they likely wouldn't end up seen by players), and given collision layers. The only reason I suggested using the tempbackground objects was because if it was done like this (which is what I would intuitively try first)...
[Action bg0] -> [bg1]
[Action bg1] -> [bg2]
[Action bg2] -> [bg3]
[Action bg3] -> [bg0]
...it wouldn't work because, line by line, bg0 would just change to bg1, then the next line would immediately change it to bg2 after that, etc., and the backgrounds would all end up being bg0. The temp objects prevent this issue.
Good luck!