OK, after complaining on this compatibility issue of FPC-dev mail list, another solution was proposed to me... FreePascal allows to define "raw" interfaces - interfaces which do not include IUnknown by default. Side effect of these interfaces is that automatic management (reference counting) of these interfaces is disabled, but it's really a plus in our cituation.

So, final result looks like this (again for Ultra's case):
[pascal][background=#FFFFFF][normal=#000000][number=#0000FF][string=#0000FF][comment=#248F24][reserved=#000000]// MyClass.pas
type
{$INTERFACES CORBA}
IMyClass = interface
procedure DoSomething; cdecl;
end;
{$INTERFACES DEFAULT}

// pasdll.dpr
procedure InitDll(const MyClass: IMyClass); cdecl;
begin
MyClass.DoSomething;
end;

exports
InitDll name 'InitDll';
[/pascal]
But remember this will not work in Delphi!