How do collagen fillers show up on scans? Had an MRI recently after having Radiesse in my temples and jawline 20 months ago.

The calcium particles that compose Radiesse stimulate collagen production.  It is these particles, not the collagen itself, that can be visible on radiographs such as an MRI.

Recently injected Radiesse can be seen on plain x-ray because of radio-dense calcium content. 
CT scan also could easily detect calcium - containing filler for the same reason.
On MRI , however, calcium is not seen readily beacuse calcium has zero spin, no intrinsic MRI signal  and calcifications contain no water.
Your MRI reports layered collagen in the areas frequently injected with fillers. If you notice, the Radiologist suggested the possibility of either Radiesse or Sculptra, but not Juvederm, Belotero or Restylane, because these two are the only biostimulant fillers used in the US with the long term effect depending on collagen stimulation.Radiesse and Sculptra are completely different fillers - one containing calcium and one without. So,  your Radiologist commented on the effect of particular filler and not on its identity.  
Calcium containing Radiesse can be detected on ultrasound because calcium is quite echogenic; other fillers would present as low density structures.
In summary, X-ray, CT scan and ultrasound could detect calcium containing fillers. MRI may detect only the effect of very selected fillers. None of these tests could identify a biodegradable filler by its name.