I haven't had to do this but I'm thinking I would plan to Airport B then Airport A. At airport B change details to VFR. In remarks note that you're doing an instrument approach at B or just leave it and explain to ATC in flight.
For those who think the concept is somehow unsafe, it can be common in mountainous area to be able to safely fly VFR once below the cloud layer to places that you simply can't get into IFR.
Edit: Put the more complicated plan in the filed flight plan, if you get to B or close to B and see that A is clear just change to VFR and continue. If you do it the other way, filing the simple plan direct to A and then have to change to doing the instrument approach at B you have more explaining to do over the radio and the controllers will be less prepared for what you want to do.