MtoA 1.2.7.0 includes an enhancement (“standin as attribute group”) that allows you to use any mesh as a standin. So instead of the aiStandin node, you can have a proxy geometry to represent your standin.

For example, here’s a mesh shape that loads an animated ASS sequence exported from Softimage:

Note that in the shape Attribute Editor, the Arnold Translator is set to procedural.

This feature uses the mesh bounding from Maya, so it’s possible your standin maybe clipped. You can work around this either by disabling Defer Procedural Load, or by using the User Options to set the procedural min and max (for example: min -1 -1 -1 max 1 1 1).

Hat tip: Alf