I've never heard about anyone doing this in EPiServer 6 and I'm pretty sure that it's not a supported case. You can create a custom property and manage serialization yourself but I recommend upgrading to EPiServer 7 where there is native functionality for this need with blocks as complex properties.
I created a custom property of type PropertyDataCollection and added following properties in the collection
I have observed the for PropertyLongString and PropertyXHTML editor the display name is not rendering in edit mode.
Is it a known issue or have anyone experienced it?