the 16GB DDR3 SO-DIMMs from Intelligent Memory (www.intelligentmemory.com) are orderable, price around $300 each, but the problem is: They most-likely won't boot on Apple systems due to a software-issue
The root-cause is going back to Intel.
Back in year 2007 the JEDEC committee created the DDR3 memory standard. Intel was part of that team. The JEDEC DDR3 standard defines the exact functionality, pinouts and possible chip-capacities of DDR3 components. Per this specification called JESD79-3 a DDR3 memory-chip can have up to 8 Gigabit (= 1GB per chip!). The Intelligent Memory 16GB UDIMM and SO-DIMM modules are made of 16 pcs of these chips.
8GB unbuffered DIMM / SO-DIMM modules are made of 16 Chips of 4Gbit each
16GB unbuffered DIMM / SO-DIMM modules are made of 16 Chips of 8Gbit each
Since Intel was part of the JEDEC group defining this DDR3 standard, it could be expected that Intel also supports all memory capacities on their CPUs -> up to max 8 Gigabit per DRAM-IC, up to 16GB per UDIMM/SO-DIMM.
From a hardware point-of-view, a 4 Gigabit DDR3 chip uses the same physical address and control lines as a 8 Gigabit DDR3 chip. Therefore 'in theory' any CPU that can take unbuffered DIMM or SO-DIMM modules with 8GB should also have no issue to take 16GB modules.
But this is just the hardware side!
Now let us look at the software-side of the story:
When you start your system, it begins loading the BIOS (on Apple it is called UEFI) from a Flash-chip on the board and executes it. One of the first parts of this BIOS/EFI is the Memory Reference Code, short called MRC. The MRC has the job to read out the SPD-data (serial presence detect information) from the memory modules installed on the board. Byte #4 of the SPD-data contains the memory component density.
The MRC software has a limitation exactly at this point. If it reads SPD Byte #4 and finds that there are memory-chips on the module with a capacity larger than 4 Gigabit per chip, it does not know what to do. The programmer of the MRC just did not include any code to handle 8 Gigabit DDR3 chips ---> the system hangs during boot!
This issue is reproducable with about every Intel CPU based system. Only for the Atom C2000, codename 'Avoton', Intel has fixed this.
Intel would eventually do a modification of the MRC code when enough people start complaining to them. If the company Apple itself talked to Intel and wishes to get this changed, it will most-likely be checked and corrected very quickly, a new BIOS or UEFI would be released and every Apple user could use 16GB modules.
But how can we push Apple to look at this? Maybe Apple is able to change the memory reference code on their own, even without having to ask for Intels help?
FYI: The famous company ASUS has done the modification of the Memory Reference Code on their own for one of their motherboards (X79 with Ivybridge E Intel i7 CPU) and reported that they are working fine now. This shows that it is possible in general!