What is it that prevents using a 3rd party SSD in a 2015 rMBP? I bought an Early 2015 MacBook Pro Retina model with the 128GB SSD and have been wondering what to do if I need more disk space.. then I found out that it's currently not quite possible to just stick any connector-wise compatible drive in place of current drive as an means of upgrading the storage.

OWC, Transcend etc sell nothing, Apple does not sell anything (at least not without installation at an ASP), however it seems eBay has some Far-East sellers that have some kind of adapters which allow for 3rd party SSDs to be attached, but they seem to have various limitations as some will not make the new drive bootable, some will boot, but do not support passwords etc leading to believe they just emulate some bit of the original drive but not all.

My question is - what is it exactly that causes this incompatibility? Is there some kind of white-list in the EFI firmware that allows only specific SSDs to work? Or is there perhaps some actual additional handshake/encryption/protocol that happens during the EFI boot that helps "to secure" the boot-up process to actually justify this incompatibility?
The reason for the "incompatibility" is simple: there is no SATA-based SSD in this Mac. Apple is transitioning away from SATA-Based SSD drives and moving towards PCIe based SSD drives.

I thought there were a couple of manufacturers that had upgrade parts for PCIe SSDs and a quick search for "MacBook Pro PCIe SSD" should yield some results for you.