Yes, they can and no doubt have been, but it's uncommon and not normal at all. Likely more the folk/jug band style than actual Bluegrass. I have liked BG since a kid and even played some Bluegrass, have friends and family who have been professional players (one was Bill Monroe, who thought we were related--but could't stand his singing, but never told him that), seen no telling how many bands, and I don't recall seeing a washboard used. Never saw an egg shaker used either. The bass is really more percussion than "musical" as in other styles. Which is one reason I never liked playing it in a BG setting. I suspect even finding a pic online of a Bluegrass band with a WB could happen, but only after passing over no telling how many pages without. Not trying to be argumentative, just pointing out it's very rare when/if it happens. I'm sure somebody does it, but it's very atypical. They are always present in Zydeco bands. Those are great. However and whenever it gets used, if it brings joy to people, bring two!