I use G-Code G4 P1 to take snapshots with external DSLR. In Slic3er its setup up in After Layerchange G-Code area.
This causes Octolapse to take a snapshot with every move to the purgetower
MK3 FW: 3.5.1
MMU2 FW: 1.0.3
Slic3r PE: 1.41.2
Setup: Using G-Code