I believe if RVR is not available, you have to convert reported visibility into RVR, using the conversion factor as stated in JAR-OPS for the prevailing conditions (night/day. full approach lights or not).
Then the approach ban would apply, so getting past the outer marker/equivalent point is the tricky bit, after that...if you have your required visual cues, land!
Can I start a visual approach if RVR is 300 meters? I don't know, will ATC even clear me for a visual approach? Probably not!
Regards,
Ziggy