Could I use a sous vide rack for cooking lasagna in boiling water?
There are no metals used in cooking that can go in hot water but not boiling water.
While metal racks for sous vide aren't meant to contact food directly they're still cooking equipment, and should still be made of food-safe materials (e.g. common culinary grades of stainless steel).
From a safety point of view I'd have no concerns at all.
Whether these racks would be much help is another matter.  The upper picture shows the lasagne sheets sitting on flat shelves.  I'd expect them to stick a bit; this is one time oil might help but you'd have to apply it to each layer of the rack.  The second picture only has a few slots available, so the sheets would be prone to sticking together.
A stainless toast rack (example from Amazon US, not a recommendation except it's about the right shape) would also work.  Do be sure to buy one that's stainless steel and not chrome plated; the plating tends to flake off at the welds before long
You can cook pasta slightly below boiling, especially when you're only par-cooking as in lasagne.  What I do if I need to precook lasagne is this case is: fill a jug (sized so the sheets sit on end) with boiling water; add the sheets one by one to make sure plenty of water gets between them; briefly microwave (perhaps 2x1 minute bursts).  Par-cooked like this they stick a little to each other but no problematically so.