This is what I've found.
I think there's no question that the names are legal at the binary level in COM, since a COM interface’s name is its IID and the text name is just documentation.
On the .NET side, the relevant specification is the Common Language Infrastructure specification (ECMA-335, http://www.ecma-international.org/publications/standards/Ecma-335.htm.) I wonder whether .NET or Mono add their own restrictions on top – to do so would reduce interoperability, but this is the real world.
Section 8.5.1 covers valid type names in the Common Type System, and simply says that names are compared using code points. Odd that it says nothing about the composition of a name, only how names are compared. This section is paraphrased by MSDN at http://msdn.microsoft.com/en-us/library/exy17tbw%28v=VS.85%29.aspx, which says that the only two restrictions are (1) type names are "encoded as strings of Unicode (16-bit) characters", and (2) they can't contain an embedded 0x0000.
I've quoted the bit about 16-bit Unicode, rather than paraphrase it, because it uses imprecise language. Presumably the author of that page meant UTF-16. In any case, ECMA-335 specifies byte-by-byte comparison, and makes no mention of Unicode (regarding type names), and neither does it prohibit embedded zeros. Perhaps .NET has deviated from the CTS here, although I doubt it. More likely, the author of this MSDN page was thinking about programming languages when he wrote it.
The Common Language Specification (also defined in ECMA-335) defines the rules for identifiers in source code. Identifiers aren't directly relevant to my question because my internal type names never appear in source code, but I looked into it anway. The CLS is a subset of the CTS, and as such its restrictions aren’t necessarily part of the broader CTS. CLS Rule 4 says that identifiers must follow the rules of Annex 7 of Technical Report 15 of the Unicode Standard 3.0 - see http://www.unicode.org/reports/tr15/tr15-18.html. That document too is a little vague, in that it refers to "other letter" and "connector punctuations" but doesn't define them. This helped: http://notes.jschutz.net/topics/unicode/.
Section 8.5.1 of the ECMA spec includes a non-normative note that a CLS consumer (such as C# or the Visual Studio type browser, I suppose) “need not consume types that violate CLS Rule 4.” My proposed interface names do violate this Rule 4. This note seems to imply that a valid type may have a name that violates rule 4, and that a CLS consumer should either accept the rogue name or safely ignore it. (The Visual Studio type browser displays it without complaint.)
So my proposed type names are generally illegal in source code. But note that section 10.1 (about identifiers in the CLS) says “Since its rules apply only to items exported to other languages, private members or types that aren’t exported from an assembly can use any names they choose.”
I conclude that it's safe to use the characters #@! in my type names as long as they remain in the binary domain and never need appear in source code nor outside the assembly. And in fact they're never used outside the COM server.
A word about future-proofing... The CTS pretty much has nothing to say about the composition of type names, despite having a section called “Valid names” (section 8.5.1). They might change that in the future, but this broad and liberal specification has invited us all to do what we like. If the CTS designers had wanted to leave room for change then surely they would have built in some provision for that, or at least been less generous.
