Instant Scenery (aka "IS"), like Airprot Facilitator X (aka "AFX") and Airport Studio (aka "AS") displays scenery objects in FS at run time via OpenGL (aka "OGL")
independent of the FSX underlying Windows DirectX sub-system.
Reportedly, that OGL rendering process is less efficient than DirectX and may not in its current form be able to "keep up" with the more desirable FPS rate offered via DirectX (which has its own throughput problems, of course); thus, in theory, OGL is suited better for for 'static' scenery object display more so than for 'moving' objects.
IIUC, legacy pre-FSX methods of "dynamic" scenery object display used non-MDL, Assembler-based methods of "library" object transformation on defined "tracks", or they used an alternate form of AI aircraft object under the control of a 3rd party module (*.DLL).
It is apparent that these methods still work in FSX as seen in the ongoing work in SCASM and/or Assembler by Oliver Pabst (AFAIK, via the underlying VistaMare software code modules) currently branded as AES/AESLite exclusively under the auspices of Aerosoft.
A notable example of this methodology is the animated trains seen in Aerosoft's "
Approaching Innsbruck" package for FS9 and FSX.
Of course, both you and Arno are more familiar with some of the "newer" MDL-based methods that FSX now offers for animated "scenery" objects via 3D geometry key-framing.
The simConnect object "injection" process, and the Living World methods are also options, but are not well documented in the SDK.
Hope this info helps !
GaryGB