Map part of composite PK to composite FK

My data model consists of items with a history. I'll call a point in time an "instant"; all tables therefore have an "instant_id" that specifies how that item was configured at that instant. The "instant_id" is rolled into a composite primary key for all tables. Imagine the following example:

Table Computer
============
PK int       instant_id    <-- Shared id
PK int       computer_id   <-- Child id
int computer_type_id    <-- Parent id
varchar      foo

Table ComputerType
==================
PK int  instant_id      <--   Shared id
PK int  computer_type_id   <--  Parent id
varchar bar

There is a foreign key in Computer mapping (instant_id, computer_type_id) to the ComputerType primary key.

We use something like

@Embeddable ComputerId {
   @Column(name='instant_id', nullable=false) int instant_id,
   @Column(name='computer_id', nullable=false) int computer_id
}

Then:

Computer {
   @EmbeddedId ComputerId id;

   @MapsId('instant_id')
   @ManyToOne
   @JoinColumns({
       @JoinColumn(name='instant_id',...),
       @JoinColumn(name='computer_type_id',...)
   })
   ComputerType computerType;
}

No matter how I combine MapsId with JoinColumns, I can't seem to get this to work. Any ideas?