Cysteine was an autocorrect typo on my part, apologies. I would still be very concerned that making any changes to your cystine molecule will change or abrogate binding in ways that you can’t predict (because you don’t know about how it binds). Especially if the changes you are making are for increasing solubility, since this would normally mean making the molecule more polar, and polarity changes like that could drastically alter binding. IMO, any results you might get from such a molecule would not really be that meaningful in terms of what you are trying to look at.
Do you know how much more cystine you can get into solution at 1% DMSO and how much excess you will have compared to enzyme concentration? It might be enough to get crystals. Have you been able to get crystals of the protein without cystine bound? I assume you probably have. Do you know if the structure changes when it binds to cystine, and could you possibly do any in silico modelling to get an idea of where it might bind?