If I'm not wrong somewhere, adding this in /system/etc/microg.xml :
<string name="location_online_source" value="positon" />
should select Positon as the default location online source. This does not seem to work : the location notification still appears after clearing microG storage or doing a factory reset, and no location source is selected in microG settings.
microG version : v0.3.10.250932
OS : iodéOS 6.9