Hmm? If I'm following what you mean, that's not the P2P rules, it's the
Unserialize code, in particular:

  compat/assumptions.h:52:static_assert(sizeof(int) == 4, "32-bit int assumed");

  serialize.h:289:uint64_t ReadCompactSize(Stream& is)

  serialize.h-679-template<typename Stream, unsigned int N, typename T, typename V>
  serialize.h-680-void Unserialize_impl(Stream& is, prevector<N, T>& v, const V&)
  serialize.h-681-{
  serialize.h-682-    v.clear();
  serialize.h:683:    unsigned int nSize = ReadCompactSize(is);

  (and other Unserialize_impl implementations)

However, ReadCompactSize throws "size too large" if the return value is
greater than MAX_SIZE == 0x02000000 =~ 33.5M, which prior to the implicit
cast to 32 bits in Unserialize_impl. And it looks like that check's been
there since Satoshi...

So as far as I can see, that encoding's just unsupported/invalid, rather
than equivalent/non-canonical?

Cheers,
aj