May I suggest contributing Khmer characters to the DejaVu open-source font project at SourceForge?
Home page is http://dejavu.sourceforge.net/wiki/index.php/Main_Page
To my eye, KhmerOS Freehand has the most open and round design that would be most compatible with DejaVu San's overall conception. However the stroke on that font is much lighter than DejaVu Sans in general.
In terms of stroke width, perhaps KhmerOS Battambang or KhmerOS Bokor would be the closest match. They might be considered more appropriate for DejaVu Serif, however.
(As it happens, I use DejaVu Sans and DejaVu Sans Mono because of how readable they are on-screen and do not even have DejaVu Serif installed, so I can't speak to its merits.)
If the license of your fonts and DejaVu are compatible, plese do join efforts and allow Khmer characters to be accessed by the wider community.
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