An CLIVirtual attribute
Just as you can use CLIMutable to store records using Entity Framework wouldn't is be nice to be able to set an CLIVirtual attribute on a record field of an ICollection type so that Entity Framework can do lazy loading. See also http://stackoverflow.com/questions/26775760/how-to-create-a-virtual-record-field-for-entity-framework-lazy-loading
Rather than forcing a strange construct onto the F# language, maybe it would be better to fix entity framework. Not a fan of EF in general - to many fundamental things missing (at least in 6, I've soured on it enough that I haven't really looked at 7) Not a fan of forcing a feature into a language because of a library.
In addition to making the property virtual, you would also have to make the record type non-sealed since EF creates dynamic proxy types that inherit from the base entity to override the virtual properties. Right now, all record types are sealed.