Have you tried zone plates instead of lenses? Feature sizes should be larger for IR than for visible light, so it's should be possible to fit enough zones to get some focusing.
To expand a bit on construction: it's possible to take a long tube, put a fresnel lens at front, pyroelectric sensor with rotating shutter at the other end. Scan by rotating it with reductors/steppers as a whole. It's also possible to cool tube with contents down to -70 with dry ice to cut on background heat radiation. Maybe it'll lessen noise in pyroelectric sensor, maybe not (and it's quite easy to buy dry ice in Moscow).