Protecting Tissue Valves
It's possible. Doctors will usually not advise patients to do something unless there is "literature" proving that it works. Some kind of randomized, double-blind, placebo controlled, long term study that proves without a doubt that the supplement or medicine will have a positive effect.
In regards to Vitamin K, Vitamin D, Magnesium, etc, there doesn't seem to be alot of mainstream "literature" supporting their positive role in specifically protecting tissue valves. So far, mostly theoretical or not specific to bovine/porcine tissue. It would make since that those supplements could protect and perhaps keep calcium away, as proven through other studies, but nothing specific to bioprosthetic valves so far.
There are ongoing studies with no published results yet, so someone must believe pretty strongly that they do have a positive effect. However, there is always a possibility that when the results are announced, that they have an opposite outcome then we would expect. That is why doctors never really advise anything, even if theoretically likely, unless there is documented proof of efficacy.