None of the Japanese shops that I know of do actual coilover conversions for the rear of a hako.
V50, RS Start, Rubber+Soul, etc all do separate strut and spring packages.
Some do sell adjustable height sleeves for the springs to help you dial in the right height, but that's not a "coilover" per se
However at least here in the states you've got a handful of companies like Troy Ermish, Ground Control, and T3 that do rear coilover conversions for the 510...
If it's do-able on the 510, it should be do-able on the hako too...
Depends if you want to change your suspension design, or if you want to keep it factory looking...
1970 Nissan Skyline GT + 1973 Datsun 240Z + 1974 VW Karmann Ghia + 1998 Toyota LandCruiser + 2006 MINI Cooper S