If you need COM interop, you don't need cross platform compatibility. You can just use the full .Net framework.
I remember working with COM on MacOS 8... And it supported remote objects, so, it'd be, in theory, possible to self inflict that pain.
I experience a physical revulsion just thinking about all the wasted years I spent with COM and DCOM.
Mono supports COM interop on Linux, too. At its most basic level, COM is really just an ABI, and there's nothing Windows-specific about it (see also: XPCOM).