hi, i looked into this same topic some time back... here's what u should do...
prepare 2 versions of your textures... one normal and one 'bloodied'. then in your class CfgVehicles section, place the following lines under the class of your unit...
wounds[]={"\mysoldier\back.pac","\mysoldier\backwounded.pac",
"\mysoldier\front.pac","\mysoldier\frontwounded.pac",
"\mysoldier\facefront.pac","\mysoldier\facefrontwounded.pac"};
continue adding in the lines for every texture that u want 'bloodied'...
enjoy !!!