Circling is a demanding exercise and should be approached with meticulous planning - there's very little room for error that close to the ground.
1.My first thought was why would you want to make a circling approach to an airfield that MUST be in LVP's (RVR <600m)? The airfield probably wouldn't allow it.
2. What's the circling minima? You cannot go below 1000' ARTE as the RVR is below minima for the instrument runway in use (Approach BAN- automatic go-around?). You cannot leave circling minima until the landing threshold/touchdown area is visually acquired and maintained. With RVR at 1500m (approx 1 nm) it's unlikely your going to see it AND maintain it as you descend. YOU are responsible for obstacle clearance so be extremely careful about descening on base leg.
3. Remember PAN OPS circling for Cat C requires you to remain with 4.2nm of threshold/abeam. Taking into account you turn radius, at 1000' agl you will be at just about the limit of that to establish on a 3 mile final.
Those are my thoughts - I'm going to think about it some more
Legally, I think you can. If the fog split was threshold 19 foggy, runway 01 9999's then it would
probably be ok. Anything else becomes an Airmanship/Command decision.
I'll try to find the legal bits.
A4