Yes, you maintain instrument currency by category, not class. 61.57(c) only mentions class in the glider-specific requirements in (c)(3). The FAA doesn't mention it at all in related legal interpretations.
Note this wording from 61.57(c)(2) (emphasis mine):
A pilot may accomplish the requirements in paragraph (c)(1) of this
section in a full flight simulator, flight training device, or
aviation training device provided the device represents the category
of aircraft for the instrument rating privileges to be maintained and
the pilot performs the tasks and iterations in simulated instrument
conditions. A person may complete the instrument experience in any
combination of an aircraft, full flight simulator, flight training
device, or aviation training device.
See also this legal interpretation, Griffith (2008) (emphasis mine):
Generally, all appropriate endorsements for passing the instrument
proficiency check, which would be otherwise given if the check were
passed in an aircraft, can also be given when one passes the check by
using an FTD, as long as the FTD represents the appropriate aircraft
category for the instrument privileges sought.
It's true that you have to pass an instrument checkride (or at least some specific tasks) in a class before you can exercise those privileges in that class, but that's a one-time thing that doesn't affect how you maintain currency. See this question for more details.