FS#2699 - HatchDialog: Pattern list based on document unit instead of measurement system
Andrew,
Tested on 3.27.6 Win7 32bit msi, replicated on 3.32.4, probably OS independent.
A bit similar as linetypes before 
... ( FS#2622 )
It seems that the list of patterns in the Hatching dialog is based on whether the document unit is considered to be metric or imperial.
Disregarding this is ambiguous for RS::Microinch (FS#2541) but that unit is probably best handled as 10-based like all metric units.
We would expect that the list is according the document measurement system.
It is not: Current implementation is whether doc unit isMetric or not.
But there is more to it:
With a custom pattern installed in the metric folder,
with a new empty document in mm, with the measurement system set to imperial,
the (metric) pattern is listed in the pattern drop-down list of the hatching dialog.
The preview is empty and the resulting patterning is also empty.
However, on saving such a file and after reloading the patterning is shown.
Pattern type is ‘from entity’ ...
... And can thus not be used for creating a new Hatch with that pattern.
Somewhere down the line the pattern is correctly sourced from the
RPatternListMetric resource list and stored in the DXF.
Background:
For a ready to donate Zig-Zag pocketing Add-On I need an available, measurement system independent and immutable custom pattern.
Exploiting the QCAD hatching engine to do a lot of the work.
But it seems I can not generate a virtual custom RPattern (Q 11971).
I must thus rely on an installed pattern file. 
Identical for both systems but also mutable and removable ...
... The problem is retrieving that with 100% certainty.
Regards,
CVH