Tricky!
According to this 
foot:conditional=no @ winter
would be a little more accurate.

I have not found a tagging scheme that takes into account that this only applies as long as a convoy is traveling.
My suggestion is to describe this in a note:

foot:conditional:note=Forbudt for gående ved kolonnekjøring, ikke ellers. Det blir kjørt 3 kolonner til Nordkapp hver dag på vinteren

foot:conditional:note:en=
Pedestrians are prohibited when convoys are driving. There are 3 convoys to the North Cape every day in winter.

PS: No wonder I don't remember a sign, it just seems to get turned around in the summer :-)