I think that any DLL that touches or uses 16 bit API is going to fail when loaded high, this probably includes most of the OS/2 system including desktop and networking.
To experiment just use highmem -b pmwp.dll (or whatever DLL you want to test). You can also try just loading the code or data high, run highmem -? to get all the options. When things go wrong use highmem -u pmwp.dll to unmark it. Make sure you have an alternate method of booting and don't be surprised if the system traps.