I’ve been working on a custom rule to constrain the minimum silkscreen width, I had no luck with some of the sample rules already posted. This seems to do the job, although so far I’m not able to assign the severity of this rule to warning - it is always reported as an error:
(rule "Minimum silkscreen line width"
(severity warning)
(constraint assertion "A.Line_Width >= 0.15mm")
(condition "A.Layer == '?.Silkscreen' && A.Type != 'text'"))