You might be able to do something with the
local_movement_speed
province modifier, if you rearrange the map to support it.
Imagine provinces A-B-C, where B is your land bridge and is
only connected to A and C. Set up a recurring
province_event
which applies local_movement_speed -10000% to B during winter and +10000% during summer. Then, in theory, passage would be impossible in winter and near-instantaneous during summer.
Note that armies will still be able to move
into B at a normal speed during winter - they just won't be able to move out. So, you probably want to provide a very generous supply limit in B so the AI doesn't accidentally kill itself. Similarly, the ability to build anything in B might end up being very overpowered. So, if I was doing this, I would model this province B as an administrative concept rather than an actual province - empty and useless, except to facilitate the land bridge.
Disclaimer: This is entirely theoretical - I have no idea if it will work in practice!
EDIT: To avoid the situation where hostile armies can approach from A and C to fight in B, it may be better to structure this kind of seasonal passage as A-B1-B2-C.
EDIT2: As you might have gathered, I'm thinking about this from the perspective of mountain passes rather than strait crossings (and I suspect OP intended the latter). The only additional wrinkle with strait crossings is you'd need to put the special crossing provinces inside an impassible sea zone (or similar), to prevent anyone from docking ships there, or landing troops, etc. This seems possible - as long as the game engine permits strait crossings to cross
multiple sea zones (ie. the two linked provinces don't have to be adjacent to the same sea zone), and I have no idea whether we can do that.