Can you use a full bar rest for any time signature? If you can't, which rest would you use to cover a whole bar with a time signature of 9/4?
Yes, it works for any time sig. Think about it, if there's a rest in a bar, but no notes, it's pretty well self-explanatory.
About the only times a semibreve rest isn't used for a whole bar are 4/2 time, and shorter that 3/16 time, both quite rarely found in pieces.
As mentioned by Lawrence Payne in an answer to a similar question, it makes sense, and is technically correct, to place the rest sign centrally in the bar, rather than at the beginning, which is where a note of the same value would normally go.