Even if AF routinely allowed for OLCI, it would be a bit risky to have travel plans premised on this as anything could happen on the day to make this not possible. I would have thought that the choice is either to reroute via elsewhere for a through ticket (the safest option) or book with plenty of buffer time at LHR so as to be able to go landside, if need be. What is the issue that makes you wary of a landside LHR connection? Queues at immigration?

Also, have you considered VS to LHR connecting to AF? It should be possible to get a through ticket. (correction: timings don't work with VS)