I've managed to get this working, so putting my notes here in case it is any use to anyone else, or helps to fix the underlying issue.
As per my notes above, I've got Visual Studio 2022 Community installed with the C/C++ compiler set up, I'm not sure this is a requirement as such for Opusmodus itself, but a compiler appears to be needed to build CLM and I had it installed already.
My starting point was where I'd got to before, but for clarity:
1. In the Opusmodus user folder, inside the CLM folder, create two new folders, bin and libclm
For me, the Opusmodus folder is located here: C:\Users\username\Opusmodus, so the new folders are as follows:
C:\Users\username\Opusmodus\CLM\bin
C:\Users\username\Opusmodus\CLM\libclm
2. In Opusmodus, uncomment the contents of 'Load CLM Instruments.lisp'
3. Run 'Evaluate All' - At this point an error will occur in the Opusmodus listener window, ignore it for now.
This step is required to make sure Opusmodus copies the libclm-o64.dll into the libclm folder.
4. Open a 'Developer Command Prompt for VS 2022'
5. Change directory in the CMD window to C:\Users\username\Opusmodus\CLM\libclm
6. Run: dumpbin.exe /exports libclm-o64.dll
This will output information about libclm-o64.dll to the terminal window, use this information to create a libclm-o64.def file containing the function names (generated def file attached)
This article was helpful, there are many other ways to do it with scripts etc, but I went the manual route: https://www.asawicki.info/news_1420_generating_lib_file_for_dll_library.html
7. Make sure to create the libclm-o64.def file is in the same folder as the libclm-o64.dll
8. With the CMD window still set to C:\Users\username\Opusmodus\CLM\libclm
Run: lib.exe /def:libclm-o64.def /out:libclm-o64.lib /machine:x64
This will generate the required libclm-o64.lib and libclm-o64.exp files.
9. Back in Opusmodus, run 'Evaluate All' again, assuming there have been no errors above, the instruments should build correctly.
Took a while, 10-15 minutes.
I've attached the def file as it's just a plan text file so it's easy to see what's in it, but it can be used to save having to do step 6. I haven't included the lib or exp files as these can be generated with step 8.
Hope that helps. Once the build had completed, I have tested and can now use all of the examples.
The above was done using Opusmodus 3.0.29226
libclm-o64.def