I dont think open fir makes any difference. If you are entering imc you should tell the controller you are declaring ifr (and in imc). The point being that while he has no authority to prevent you doing so both he and any other aicraft on frequency are now aware that you can no longer see and avoid. If a radar service is available chances are even if their isnt capacity for vfr traffic you will get a service.
A common mistake having declared ifr is to believe that because you are in open fir you can deviate from the declared height. You shouldnt because the controller is entitled to expect you to maintain height unless you inform him to the contrary.