Custom Binary File Formats (I'd love some help!)

[iyenal]

Retro Studios, my game dev heroes

Me also.
For example, analyse ASCII FBX format to see how it's structured. And made a custom format with XML arrangement, Unity can load them with this : http://wiki.unity3d.com/index.php?title=...Serializer . But do you want this custom format to be embedded in the Unity build ?
But if you will do a hexadecimal from scratch format, good luck.