Can attributive adjectives be modified by adverbs of frequency?

Yes. For example, always can also be used with adjectives in attributive use. The Oxford English Dictionary (under "always, adv.") gives several examples of the usage:

Compounds

C1. With adjectives in attributive use, as always-immaculate, always-planned, always-popular, always-quotable, always-ready, etc. Cf. always-on adj., ever adv. and adj. Compounds 2.

Other adverbs of frequency can be adapted similarly: the rarely-late doctor, the often-sleepy child.