I would strongly suggest not using the built-in binary serialization even if you could... and it sounds like basically can't anyway. There are plenty of alternatives for serialization, depending on your requirements. You could consider:
- JSON (e.g. with Json.NET)
- Protocol Buffers
- YAML
- XML
These have different trade-off in terms of whether you need to make the data that you're serializing separate to your normal code or whether they can serialize "in-place", the readability, efficiency etc. But all are nice and portable, not just between .NET/Xamarin implementations but also other platforms.