I have stored D. mel embryos and imaginal discs in PBS at 4 degrees after primary antibody washes. For me it worked for at least 2 or 3 days without major changes in the staining pattern.

If you are concerned that you will lose your primary during storage, you could do another fixation step to fix your primary in place, rinse and then store while you wait for your secondary. Just ensure to include correct controls. If you are not concerned  and will only store it for a few days, I think it will be fine.

I doubt you'll lose the antibody. At 4°C the binding should be stable enough for several days. Just call it very stringent washing :-) I'm using PBS+TritonX100, and never had a problem.

I have also kept embryos at 4oC in PBS+0.3% Triton after primary for two weeks and they have still stained beautifully. It is quite robust.