The issue with your dylibs no longer working under beta 6 is probably because they added a requirement for all binaries (executables and dylibs) to be code signed in that version. When you link an exe/library, the linker will add a fake code signature (that will be trusted by you local system, but not usable for distribution). That signature will be invalidated if you perform any extra operations on the generated binary afterwards though, in particular using the strip tool. And while FPC uses the linker's symbol stripping functionality when generating executables, it uses the strip tool when generating dynamic libraries (I think this is because older versions of the linker stripped too many symbols away).
So it should work if you codesign your libraries again after stripping them, either using a fake signature (see the beta 6 release notes for the command) or using a real code signing certificate. I'll look into changing the compiler as well. I'll also look into beta 7 issues (I did get a DTK in the end about two weeks ago).
Edit: to disable stripping, compile with -Xs-. That should allow your dylibs to work again.
Edit 2: it's actually listed as a known bug that strip invalidates these signatures, so I'll wait with making changes to the compiler since that suggests Apple may still fix it