FYI the feature size seems to be hardcoded to 0.5mm but I can confirm that a a 0.3mm feature size prints on silkscreen well and scans fine on boards produced by one of the cheap Chinese fabs.
The python plugin structure is fairly amenable to a bit of lightweight hacking. This thread is worth a read
I also grafted on a DataMatrix version which makes much smaller codes - 6mm x 6mm are easily possible but the text length is more limited - fine for a serial number of version control ref though.