First of all, I'd suggest developing a new method. HPLC has come a long way in 19 years!
If you really want to proceed, add 5 mL or more of water to a liter of butyl chloride and agitate to let the two phases equilibrate (the solubility of water in butyl chloride is listed in the B&J handbook as 0.08% at 20 deg C).You can pour or draw off the water-saturated butyl chloride as needed.
Getting *dry* butyl chloride is another matter. The better grades of butyl chloride are listed as "< 0.02% water", so a freshly-opened bottle should be OK. I don't know how fast that will change on exposure to atmospheric water vapor (probably not too fast, since they determine water content by Karl Fischer titration). I don't know if butyl chloride forms an azeotrope with water. If it does, you can dry it easily by distilling off the azeotrope. Drying with molecular sieive is another possibilty.
To get 50% water saturated, you blend equal volumes of each (obviously).
How much trouble you have to go to depends on the robustness of the method with respect to water saturation; it may not matter much (then again, . . .)