I am with CTx for map_c/map_t work.
I have a cunning plan for Leningrad and it will involve modifying map_c and overlaying with some plates objects with alpha channels.
You will have to use the plates objects in a sensible manner. With LODs limitation you will end up with "popping up" ice as you fly towards the plates.
Not there yet (learning GMax...and getting the summer map right) but I am sure we can get a transition from melted to half-frozen then to fully frozen. If anything I will get rid of the water and replace it with a texture(s) that look like ice.
Around Leningrad not the whole of Ladoga gets frozen and not all of the Gulf of Finland gets completely frozen. Even the Neva is not always
completely frozenHere is an old screenshot of my younger years with swampy/very shallow parts:
https://yadi.sk/i/b_cEkf6b3EHpFQ (see just at the wing root of the plane), I am sure we can get something better than all ice/no ice (like the Finsgulf winter map).
I expect to have to trade some of the textures used for summer (fields, bush areas) to get some variety in ice representation ... As long as everybody is happy with the whole thing being ready in 2028, it's all good!!!
Also I only have seven (map_c) rivers on my map, this will not be hard to fill...lots of lakes though!