I'm using mugi-urushi now, so I can answer that question @joXn asked about celiac and wheat-based urushi. My teacher thinks that it would be ok initially, because there are multiple non-wheat-containing layers of urushi on top of the mugi-urushi. But he thought over a long time, gluten could theoretically leach out as the outer layers wear down.
Historicay mugi-urushi was less likely to be made with wheat, so I think you could circumvent this entirely by using rice flour at the cost of moderately weaker strength.