lol Oh I know all about picky siamese I have 3 of them. (Blue-point, Flame-Point & Lynx Point). All of which are extrememly picky or at least were until everyone went on the same diet. Now as long as I vary the flavors everyone eats what they should.
Other than freezing her food the only other thing I can think of is a timed feeder. But then again you would be running into Oscie being there to steal food. Unless they both can be put on the same diet. Lastly would be to move her into a room of her own while you are away. Give her a room with LB and food and water, and keep Oscie out of that room while you are away. Then when you are home let her out to be with the rest of the household. Or the reverse lock up Oscie while you are gone and leave her out in the house.
We do that when we have to be gone for awhile with Max, not because of there being any dry food around, but because he grazes and a couple of mine will hoover up any scraps left. So when I need to leave extra food because we will be gone for extended periods of time, Max goes in the bedroom/bathroom where he has a window ledge to lay on, his own litter box and his own dishes, the other nine get the run of the house.
Hopefully you will find a solution that works for everyone.
Mel, Max & The Fur Gang