Pevicons have been used in science since the later 1970’s and operated in the MWIR and LWIR bands in specialist spectrographic equipment. They can operate as low as 2um. The tube needs to be configured with the correct target material and band filtering however. Whilst the Pevicon used in the Argus 1 camera may contain a target material that will respond to MWIR, its Germanium window is likely coated for LWIR only. Bill W may know more about that specific tubes spectrum sensitivity.
Even if capable of MWIR, remember that Pevicons are not famous for their sensitivity. It was not unknown to see sensitivity figures of 1.0C with enhanced versions providing nearer to 0.2C. The P8092 is specified as having a sensitivity in the LWIR band of 0.2C. With the available energy present at room temperature in the MWIR band, that could be an issue. Bill W would be my guide on Pevicon capability though and I bow to his far greater knowledge of the technology.
Fraser