CUST.Lemmings causes a Guru Meditation:
exception $FF07FE74 (vector 1069678493) at $79260011
SP: 0001FFD8, Stack: 00117926 00117720 000000C0 00000000
Emulation of procedure Custom Module/InitSound ($00117708) cannot continue.
Also, I see you mentioned not releasing the source. How about access to a private Git repository under NDA or something? I would like to port it to foobar2000 and also Cog for Mac OS X. The former, you may be equipped to do. The latter, probably not so much.
Oh, and how would you like a multi-instance capable version of StarScream? It lives in my Highly Theoretical source repository, as the last version of StarScream that Neill worked on, but never released publicly before he let me release the xSF libraries. I use it in the Windows version of foo_input_ht, but I use M68k everywhere else, because it's still fast enough, and also portable to other architectures.
You're welcome to attempt that Cog port yourself, using the interface headers in my Cog Bitbucket repository, as it does support user installed components in a subdirectory of their home directory. You can look at the full project source there to see how it's organized, but really, all it needs is:
1) Create new bundle project.
2) Remove unnecessary libraries, like Cocoa runtime.
3) Add (cog root)/Audio/Plugin.h to project, for interface #import later.
4) Add (cog root)/Playlist/PlaylistController.h if you want to be able to detect Repeat One mode, for indefinite looping control.
5) Add (cog root)/Utils/Logging.h if you want to use the @ALog() diagnostic logging function, helpful for debugging.
6) Implement decoder/meta/container interfaces in ObjC/++, in separate interface objects if you wish. Decoder does nothing but playback and track sample format, channel count, and length reporting, meta reports titles and such, container reports if a module contains subsongs and enumerates those as playlist entries. See: Dumb in (cog root)/Plugins/Dumb.
7) Build.

Use Finder or ditto to package up Delix.bundle for distribution.
Naturally, that bit will require replacing Starscream with M68k, as Cog is built for 64 bit targets.
Sounds like fun, doesn't it? If I were to port it, I would most likely package the source code in a private repository and distribute the bundle inside the player app bundle.