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It is not just osmotic factors that need balancing, but the fact that polymers and salts don't always mix well.  PEG and high salt can form 2-phase systems that don't disperse well, particularly in a crystal.  Check out Ray et al. (Biochemistry 1991, 30, 6866-6875) where Bill Ray and and others from Purdue needed to exchange  ~2.1 M ammonium sulfate in phosphoglucomutase crystals to various PEGs just to do the kind of experiments you suggest.  It can be a pain, but osmolarity matching was not the big issue.  As Bill was an excellent, old-time physical biochemist, he developed the system based on first-principles.  You may also look up the work of Pierre Douzou and the cryoenzymology.  Douzou collaborated with Petsko on some protein crystal work doing the same kind of exchange to reduce the crystal's freezing point.