I haven't opened one up, but typically, with air forks, you lower them by putting material below the piston on the air shaft, provided, they use a simple configuration of a piston sitting on top of a shaft as found in Fox Float series and Rock Shox Solo or Dual air. For 20mm drop you use 20mm of whatever under the shaft. You may be able to use a Fox or a Rock Shox travel spacer or make your own.
Marzocchi may be different; at least, their Doppio Air cartridge is enclosed and reducing travel there would require a different approach. The TST2 may have a very simple arrangement on the air piston; I'm not sure.
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