I've saved/frozen each foaling season for many years now. It has not ever put a foal in a compromised situation. The foal gets the first 3-4 hours worth, then I begin to take an ounce or two, every other hour or so. I mark the mare it came from, date, etc. I then continue to obtain some milk from the 2nd day, same deal mark mare and date. If I have a foal who needs help, then I know which is pure colostrum and the rich 2nd day milk. I keep mine from season to season and while I have only needed it once, I have been able to share this little "pot of gold" with others.
Did the same with my donks & goats......needless to say, I have a good amount of frozen milk :bgrin but, I had a separate frig for such things. The goat milk is also good for orphaned deer -- had a wildlife rescuer who often called me for that. Yes, thaw by setting the container of milk into warm water. Too much heat will kill the good stuff in the milk.
My mares have all been very good milkers and sometimes your maidens will be less so......another place to obtain a real quantity is if your foal doesn't make it. We all know how heartbreaking this is but, that mare could very well safe the life of yet another foal by saving a good quantity of colostrum from her, if she was full termed, etc.
OH, I freeze mine by putting it in a little 2oz cup (like a pill cup) and cover with plastic wrap to form a top....when it freezes this way you eliminate the air that causes the product to deteriorate...get freezer burn. If you use actual ice cube trays, after frozen put them into a plastic bag and suck the air out of it to vacuum seal as much as possible.