Another apparently prefixed adverb that does not seem to lenite is bras (intensifieur): ur vro bras-ec'hon (G).
Cf. optional (not obligatory, as apparently above) nonlenition for adjectives in coordination: ur gazeg bihan ha glas, ur waremm gouez, kompez (Press) or with a complement, etc (Press, Favereau). It seems that intensifiers are among the factors that block lenition: un vro bras-meurbet, un dra bras-abominapl, ur vougev bras-kenan (G), where the SUFFIX intensifier prevents the adjective from leniting. In fact Favereau p. 152-3 explicitly gives this for un dra mat-tre &c.