It really depends on what operations are used inside the EPS file. You could redefine for example setgray like this:
/setgray_orig /setgray load def
/setgray { 0.9 gt {1.0}{0.5} ifelse setgray_orig } bind def
and then on exit redefine it like so:
/setgray /setgray_orig load def
Now this depends on setgray being used its possible that your eps is using sethsbcolor, setrgbcolor, setcmykcolor etc. if so then you should wrap those too. Be careful with this stuff.
Please note it is not enough to define all gray as 0.5 since someone might be painting white to fill areas so that has to be accounted for too.
Ammendum: you could also just define the setgray inisde the /ostate save def and ostate restore directives then you don't need to separately unload the functionality.