I am thinking the extra travel will a) slacken the head angle
maybe not as much as you think, 650B forks will be longer A2C for the same travel, so a 140mm 26in fork is close to a 130mm 650B fork, but there can be more than 10mm variation in A2C between forks of the same travel between different manufacturers/models so without having actual A2C measurements to hand it’s hard to say, you might just end up with the same HA as you would with a 650B 120-130mm fork rather than actually slackening it.
b) raise the BB a bit to get it back to 27.5″ height
even if the fork is actually longer it won’t affect the BB height that much, depending on actual frame angles etc. it might make 5-10mm difference, but you could easily lose than in sag again, or as above due to A2C lengths you might not gain any BB height at all…
However, crack on It’ll likely be fine, the absolute worst case scenario is a slightly low BB, which you can probably learn to live with or work around.
I’m still running 26in Wheels in my 650B Flare, the BB si a bit low, but with 165mm cranks it’s not terrible, and it’s still higher than the BB on my actual 26in BTR so just proves there’s often more variation between bikes* than difference you can make with a few mm lost or gained here and there.
If it turns out terrible and unworkable you’ll just have to get some new forks and wheels (or 26+ tyres**) 😉
* When I got the Flare I did a bit of geometry comparing and worked out that there were plenty of other frames out there with 10-15mm lower BBs anyway, so losing ~15mm of BB height to my small wheels just puts it in the same ballpark as other bikes running the ‘right’ size wheels.
** Currently have some WTB 26+ Tyres on there that bring the outside diameter to within a few mm of a 650Bx 2.2-2.3, they were lovely over summer, terrible in the mud though!
