I've tried it the way you did above:

int wrap_RegisterClient2(int cl_id, char* read_ip) {
  return RegisterClient2(cl_id, read_ip);
}
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but it doesn't work, I get the same error message as in the case without the wrapper.

The problem, I believe, is that unless all the parameters, and the return type, are types that the default typemap knows how to handle, Inline::C doesn't wrap and export it. And it appears that the default typemap doesn't know how to handle int *.

So, whilst this works fine:

#! perl -slw
use strict;
#use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<'END_C',  NAME => '_test', CLEAN_AFTER_BUILD => 0;

int test( int i ) {
    return i;
}

END_C

print test( 123 );
__END__
C:\test>test
123
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This doesn't:

#! perl -slw
use strict;
#use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<'END_C',  NAME => '_test', CLEAN_AFTER_BUILD => 0;

int test( int *i ) {
    return *i;
}

END_C

print test( 123 );

__END__
C:\test>test
Warning. No Inline C functions bound to Perl in C:\test\test.pl
Check your C function definition(s) for Inline compatibility

Use of inherited AUTOLOAD for non-method main::test() is deprecated at
+ C:\test\test.pl line 12.
Can't locate auto/main/test.al in @INC (@INC contains: C:\test\_Inline
+\lib C:/Perl64/site/lib C:/Perl64/lib .) at C:\test\test.pl line 12
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There is the TYPEMAPS configuration option, but I haven't seen any (good) documentation on that. And finding the default typemap and looking at that doesn't yield many clues.