Is it appropriate (legal) to request practice instrument approaches in IMC?
Certainly you can request practice approaches in IMC on an IFR flight plan.  ATC will accommodate aircraft on a first come, first served basis. It is legal, and appropriate if ATC can accommodate it based on traffic (or noise or NOTAM restrictions).

Of course there will be some difficulty if you attempt to do practice approaches at Los Angeles (LAX) or New York (JFK). ATC would not likely accommodate this at these or similarly busy high performance aircraft airports. But again, unless it was otherwise restricted (e.g., noise abatement requirements or NOTAM restrictions) you can certainly request this type of IFR/IMC handling.