Thanks to Ayan Patra, I found the solution.
Measurement units can be changed in libreoffice/4/user/registrymodifications.xcu
For calc it was set in:
<item oor:path="/org.openoffice.Office.Calc/Layout/Other/MeasureUnit"><prop oor:name="Metric" oor:op="fuse"><value>8</value></prop></item>
For writer it's in
<item oor:path="/org.openoffice.Office.Writer/Layout/Other"><prop oor:name="MeasureUnit" oor:op="fuse"><value>8</value></prop></item>
The values as far as I tested were
millimeters: 1
centimeters: 2
inch: 8
Tested these with the gui on local machine and the oor:name in the calc config stays metric when changing to imperial units... Doesn't seem to matter thou.