There are some ways this is an easy question and there are some ways it is not.
It is often a perfectly useful approximation to hold that hydrostatic pressure does not alter the distribution of stable isotopes in a system at equilibrium. This is not strictly true; one can write Gibbs free energy in terms of pressure and volume. If the molar volumes of two isotopologues are not equal, neither are their Gibbs free energies. In most cases, the change in molar volume that comes with isotopic substitution is so small that it is safely ignored. Hydrogen is almost sui generis in that the molar volume difference is not trivial with substitution, so it's not a safe bet to assume there's no difference.
If you are concerned about kinetic phenomena, there are circumstances where there is unquestionably a pressure effect. This is most obvious in biochemistry, where the catalytic capability of enzymes is a function of hydrostatic pressure. I haven't spent any real time thinking about whether fractional crystallization causes pressure-dependent isotopic fractionation, but I would not be surprised in the least if so. Someone here probably knows that kind of thing.
Department of Earth and Atmospheric Sciences
Bloomington IN 47405