Im on that forum often. havent found anything about librem and coreboot on the 15v3, but i know at least a couple people at purism play with qubes. update: i found kakarots thread 
also, there are purism entries in the mailing list that didnt make the HCL. ill bug them about it.
one of my concerns is AEM (anti evil maid). Theres no mention of a TPM on the product page, and without that, of course, AEM wont work. the fallback would be full disk encryption either with luks from within coreboot, or if sed if its supported. or opal to write lock part of the drive.
this post also disturbs me as its the model i was looking at. not having vt-d is a deal breaker for me. VT-d not enabled in BIOS (coreboot)