After playing around (unsuccessfully with SwitchResX) I checked my old Backups of the situation a day prior to Upgrade to Yosemite.
Interesting stuff: I restored from an earlier TimeMachine Backup the specific Display Profile File (used on Maverick & Yosemite for 34UM95 --> checked with ioreg command) to my MacBookPro (System/Libraries/Displays/Overrides/DisplayVendorID-1e6d) and so far - even as Dualscreen - my 34UM95 can be used with native resolution 3440x1440 (connected to Thunderbolt). --> so this should work eventually for you guys, having a 34UM95 connected to a MacBook as second screen. In Display Settings (OS X) the resolution 3440x1440 is shown and I can choose between 60Hz (NTSC), 50 or 30Hz PAL
The File used for 34UM95 seems to be "DisplayProductID-76db" (checked on MacBookPro as well MacMini): Prior to Yosemite Update it was from 28th Feb. After Yosemite it has ChangeDate of 9th September. Internally (binary/hex coded settings) it seems to be different to the Yosemite version as well in Yosemite there is a string attribute LG ULTRAWIDE, where as the old file just keeps the same values 7789 for VendorID and DisplayProduct 30427
On my MacMini I unfortunately started TimeMachine Restore back to Maverick yesterday (still restoring). Eventually I will upgrade again to Yosemite and try to exchange this File with Maverick version.....on my MacBookPro (Late2013, 13'') it seems to work some, though....