Are you referring to a 'Myers hub' as your Rigid connector?
I've never seen diecast connectors ever given a RT listing. Steel, yes.
The newer RT connectors don't differ much: just an additional seal, AFAIK.
I've seen inspectors tolerate Greenfield/ flex ( not Sealtite ) for THWN-2 taps to HVAC condensers -- in residential situations.
In which case, might D.C. S.S. connectors also pass for your application?
BTW, at 3", DCSS fit both RMC/IMC and EMT.