So I've been working on this for a week, trying to backport fliers from current PW into 1.5.5, yes its possible. There's a lot more to it than just modifying the ECM file. You need to:
1) Copy the whole folder from models.pck for the flier
2) The ECM files also reference various .gfx files for the sparkly/glowy effects around the flier, so you need to also copy those across from gfx.pck
3) Some newer ECM files are MOXTVersion: 71, 1.5.5 only supports 67. To downgrade these, remove lines that say "AudioEventGroupEnable: 1", "ParticleBonesCount: 0" and "GfxUseFixedPoint: 0" and then obviously change the version number on the 1st line.
4) Some new SMD files are version 9, 1.5.5 only supports 8. Downgrading these isn't difficult, as the only change I could find is an additional 4 byte value in the actions array (so each array entry is action name, start frame, end frame, new value, animation filename). Couldn't find any documentation on this but its straightforward.
5) Some new STCK files are version 2, 1.5.5 only supports 1. This is the really hard one, because version 2 a) stores the segment data completely differently, and b) can "compress" quaternions by leaving off the w value, which you then calculate by making the quartenion unit length given the x, y, z values. The code to read both version 1 and version 2 is in the angelica 2.2 source code which is kicking around here. Looking at enough examples of v1 files and v2 files you can figure out how to write the old style segment data.
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